diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Order.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Order.java
index e7d5964c..aaee9efd 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Order.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Order.java
@@ -172,7 +172,7 @@ public class Order extends AcmeResource {
         URL certUrl = json.get("certificate").asURL();
         certificate = certUrl != null ? Certificate.bind(getSession(), certUrl) : null;
 
-        this.error = json.get("error").asProblem();
+        this.error = json.get("error").asProblem(getLocation());
 
         this.authorizations = json.get("authorizations").asArray().stream()
                 .map(JSON.Value::asURL)
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Problem.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Problem.java
index 66eb15cd..a46a250c 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Problem.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Problem.java
@@ -14,6 +14,7 @@
 package org.shredzone.acme4j;
 
 import java.io.Serializable;
+import java.net.URI;
 
 import org.shredzone.acme4j.util.JSON;
 
@@ -25,6 +26,7 @@ import org.shredzone.acme4j.util.JSON;
 public class Problem implements Serializable {
     private static final long serialVersionUID = -8418248862966754214L;
 
+    private final URI baseUri;
     private final JSON problem;
 
     /**
@@ -32,16 +34,20 @@ public class Problem implements Serializable {
      *
      * @param problem
      *            Problem as JSON structure
+     * @param baseUri
+     *            Document's base {@link URI} to resolve relative URIs against
      */
-    public Problem(JSON problem) {
+    public Problem(JSON problem, URI baseUri) {
         this.problem = problem;
+        this.baseUri = baseUri;
     }
 
     /**
-     * Returns the problem type.
+     * Returns the problem type. It is always an absolute URI.
      */
-    public String getType() {
-        return problem.get("type").asString();
+    public URI getType() {
+        String type = problem.get("type").asString();
+        return type != null ? baseUri.resolve(type) : null;
     }
 
     /**
@@ -51,6 +57,15 @@ public class Problem implements Serializable {
         return problem.get("detail").asString();
     }
 
+    /**
+     * Returns an URI that identifies the specific occurence of the problem. It is always
+     * an absolute URI.
+     */
+    public URI getInstance() {
+        String instance = problem.get("instance").asString();
+        return instance != null ? baseUri.resolve(instance) : null;
+    }
+
     /**
      * Returns the problem as {@link JSON} object, to access other fields.
      */
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
index 6534b769..a9b5ca50 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
@@ -43,6 +43,7 @@ import org.jose4j.base64url.Base64Url;
 import org.jose4j.jwk.PublicJsonWebKey;
 import org.jose4j.jws.JsonWebSignature;
 import org.jose4j.lang.JoseException;
+import org.shredzone.acme4j.Problem;
 import org.shredzone.acme4j.Session;
 import org.shredzone.acme4j.exception.AcmeException;
 import org.shredzone.acme4j.exception.AcmeNetworkException;
@@ -237,7 +238,10 @@ public class DefaultConnection implements Connection {
                 throw new AcmeException("HTTP " + rc + ": " + conn.getResponseMessage());
             }
 
-            throw createAcmeException(readJsonResponse());
+            Problem problem = new Problem(readJsonResponse(), conn.getURL().toURI());
+            throw createAcmeException(problem);
+        } catch (URISyntaxException ex) {
+            throw new AcmeProtocolException("Bad request URI: " + conn.getURL(), ex);
         } catch (IOException ex) {
             throw new AcmeNetworkException(ex);
         }
@@ -411,32 +415,29 @@ public class DefaultConnection implements Connection {
      * {@link AcmeServerException} or subtype will be thrown. Otherwise a generic
      * {@link AcmeException} is thrown.
      */
-    private AcmeException createAcmeException(JSON json) {
-        String type = json.get("type").asString();
-        String detail = json.get("detail").asString();
-        String error = AcmeUtils.stripErrorPrefix(type);
-
-        if (type == null) {
-            return new AcmeException(detail);
+    private AcmeException createAcmeException(Problem problem) {
+        if (problem.getType() == null) {
+            return new AcmeException(problem.getDetail());
         }
 
+        String error = AcmeUtils.stripErrorPrefix(problem.getType().toString());
+
         if ("unauthorized".equals(error)) {
-            return new AcmeUnauthorizedException(type, detail);
+            return new AcmeUnauthorizedException(problem);
         }
 
         if ("userActionRequired".equals(error)) {
-            URI instance = resolveRelative(json.get("instance").asString());
             URI tos = getLinks("terms-of-service").stream().findFirst().orElse(null);
-            return new AcmeUserActionRequiredException(type, detail, tos, toURL(instance));
+            return new AcmeUserActionRequiredException(problem, tos);
         }
 
         if ("rateLimited".equals(error)) {
             Optional<Instant> retryAfter = getRetryAfterHeader();
             Collection<URI> rateLimits = getLinks("urn:ietf:params:acme:documentation");
-            return new AcmeRateLimitExceededException(type, detail, retryAfter.orElse(null), rateLimits);
+            return new AcmeRateLimitExceededException(problem, retryAfter.orElse(null), rateLimits);
         }
 
-        return new AcmeServerException(type, detail);
+        return new AcmeServerException(problem);
     }
 
     /**
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededException.java b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededException.java
index 66792440..d29256fe 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededException.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededException.java
@@ -18,6 +18,8 @@ import java.time.Instant;
 import java.util.Collection;
 import java.util.Collections;
 
+import org.shredzone.acme4j.Problem;
+
 /**
  * An exception that is thrown when a rate limit was exceeded.
  */
@@ -30,19 +32,16 @@ public class AcmeRateLimitExceededException extends AcmeServerException {
     /**
      * Creates a new {@link AcmeRateLimitExceededException}.
      *
-     * @param type
-     *            System readable error type (here
-     *            {@code "urn:ietf:params:acme:error:rateLimited"})
-     * @param detail
-     *            Human readable error message
+     * @param problem
+     *            {@link Problem} that caused the exception
      * @param retryAfter
      *            The moment the request is expected to succeed again, may be {@code null}
      *            if not known
      * @param documents
      *            URIs pointing to documents about the rate limit that was hit
      */
-    public AcmeRateLimitExceededException(String type, String detail, Instant retryAfter, Collection<URI> documents) {
-        super(type, detail);
+    public AcmeRateLimitExceededException(Problem problem, Instant retryAfter, Collection<URI> documents) {
+        super(problem);
         this.retryAfter = retryAfter;
         this.documents =
                 documents != null ? Collections.unmodifiableCollection(documents) : null;
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeServerException.java b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeServerException.java
index 7c0010eb..c36c227f 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeServerException.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeServerException.java
@@ -13,8 +13,11 @@
  */
 package org.shredzone.acme4j.exception;
 
+import java.net.URI;
 import java.util.Objects;
 
+import org.shredzone.acme4j.Problem;
+
 /**
  * An exception that is thrown when the ACME server returned an error. It contains
  * further details of the cause.
@@ -22,27 +25,31 @@ import java.util.Objects;
 public class AcmeServerException extends AcmeException {
     private static final long serialVersionUID = 5971622508467042792L;
 
-    private final String type;
+    private final Problem problem;
 
     /**
      * Creates a new {@link AcmeServerException}.
      *
-     * @param type
-     *            System readable error type (e.g.
-     *            {@code "urn:ietf:params:acme:error:malformed"})
-     * @param detail
-     *            Human readable error message
+     * @param problem
+     *            {@link Problem} that caused the exception
      */
-    public AcmeServerException(String type, String detail) {
-        super(detail);
-        this.type = Objects.requireNonNull(type, "type");
+    public AcmeServerException(Problem problem) {
+        super(Objects.requireNonNull(problem).getDetail());
+        this.problem = problem;
     }
 
     /**
      * Returns the error type.
      */
-    public String getType() {
-        return type;
+    public URI getType() {
+        return problem.getType();
+    }
+
+    /**
+     * Returns the {@link Problem} that caused the exception
+     */
+    public Problem getProblem() {
+        return problem;
     }
 
 }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUnauthorizedException.java b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUnauthorizedException.java
index 651d807e..11d8fec8 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUnauthorizedException.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUnauthorizedException.java
@@ -13,6 +13,8 @@
  */
 package org.shredzone.acme4j.exception;
 
+import org.shredzone.acme4j.Problem;
+
 /**
  * An exception that is thrown when the client is not authorized. The details will give
  * an explanation for the reasons (e.g. "client not on a whitelist").
@@ -23,14 +25,11 @@ public class AcmeUnauthorizedException extends AcmeServerException {
     /**
      * Creates a new {@link AcmeUnauthorizedException}.
      *
-     * @param type
-     *            System readable error type (here
-     *            {@code "urn:ietf:params:acme:error:unauthorized"})
-     * @param detail
-     *            Human readable error message
+     * @param problem
+     *            {@link Problem} that caused the exception
      */
-    public AcmeUnauthorizedException(String type, String detail) {
-        super(type, detail);
+    public AcmeUnauthorizedException(Problem problem) {
+        super(problem);
     }
 
 }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredException.java b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredException.java
index 6a5dfee6..7f6ea698 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredException.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredException.java
@@ -13,9 +13,12 @@
  */
 package org.shredzone.acme4j.exception;
 
+import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URL;
 
+import org.shredzone.acme4j.Problem;
+
 /**
  * An exception that is thrown when the user is required to take action as indicated.
  */
@@ -23,26 +26,18 @@ public class AcmeUserActionRequiredException extends AcmeServerException {
     private static final long serialVersionUID = 7719055447283858352L;
 
     private final URI tosUri;
-    private final URL instance;
 
     /**
      * Creates a new {@link AcmeUserActionRequiredException}.
      *
-     * @param type
-     *            System readable error type (here
-     *            {@code "urn:ietf:params:acme:error:userActionRequired"})
-     * @param detail
-     *            Human readable error message
+     * @param problem
+     *            {@link Problem} that caused the exception
      * @param tosUri
      *            {@link URI} of the terms-of-service document to accept
-     * @param instance
-     *            {@link URL} to be visited by a human, showing instructions for how to
-     *            agree to the terms and conditions.
      */
-    public AcmeUserActionRequiredException(String type, String detail, URI tosUri, URL instance) {
-        super(type, detail);
+    public AcmeUserActionRequiredException(Problem problem, URI tosUri) {
+        super(problem);
         this.tosUri = tosUri;
-        this.instance = instance;
     }
 
     /**
@@ -58,7 +53,13 @@ public class AcmeUserActionRequiredException extends AcmeServerException {
      * or {@code null} if the server did not provide such a link.
      */
     public URL getInstance() {
-        return instance;
+        try {
+            URI instance = getProblem().getInstance();
+            return instance != null ? instance.toURL() : null;
+        } catch (MalformedURLException ex) {
+            throw new AcmeProtocolException(
+                    "Bad instance URL: " + getProblem().getInstance().toString(), ex);
+        }
     }
 
 }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/util/JSON.java b/acme4j-client/src/main/java/org/shredzone/acme4j/util/JSON.java
index 9df71ade..a2a6b6cf 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/util/JSON.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/util/JSON.java
@@ -306,14 +306,20 @@ public final class JSON implements Serializable {
         /**
          * Returns the value as {@link Problem}.
          *
+         * @param baseUrl
+         *            Base {@link URL} to resolve relative links against
          * @return {@link Problem}, or {@code null} if the value was not set.
          */
-        public Problem asProblem() {
+        public Problem asProblem(URL baseUrl) {
             if (val == null) {
                 return null;
             }
 
-            return new Problem(asObject());
+            try {
+                return new Problem(asObject(), baseUrl.toURI());
+            } catch (URISyntaxException ex) {
+                throw new IllegalArgumentException("Bad base URL", ex);
+            }
         }
 
         /**
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/OrderTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/OrderTest.java
index 0abf4a78..2353a81f 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/OrderTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/OrderTest.java
@@ -19,6 +19,7 @@ import static org.shredzone.acme4j.util.AcmeUtils.parseTimestamp;
 import static org.shredzone.acme4j.util.TestUtils.*;
 
 import java.net.HttpURLConnection;
+import java.net.URI;
 import java.net.URL;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -76,7 +77,7 @@ public class OrderTest {
         assertThat(order.getCsr(), is(csr));
 
         assertThat(order.getError(), is(notNullValue()));
-        assertThat(order.getError().getType(), is("urn:ietf:params:acme:error:connection"));
+        assertThat(order.getError().getType(), is(URI.create("urn:ietf:params:acme:error:connection")));
         assertThat(order.getError().getDetail(), is("connection refused"));
 
         List<Authorization> auths = order.getAuthorizations();
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/ProblemTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/ProblemTest.java
index edad3c9a..29cd4126 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/ProblemTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/ProblemTest.java
@@ -17,6 +17,8 @@ import static org.hamcrest.Matchers.is;
 import static org.junit.Assert.assertThat;
 import static uk.co.datumedge.hamcrest.json.SameJSONAs.sameJSONAs;
 
+import java.net.URI;
+
 import org.junit.Test;
 import org.shredzone.acme4j.util.JSON;
 import org.shredzone.acme4j.util.TestUtils;
@@ -28,12 +30,14 @@ public class ProblemTest {
 
     @Test
     public void testProblem() {
+        URI baseUri = URI.create("https://example.com/acme/1");
         JSON original = TestUtils.getJSON("problem");
 
-        Problem problem = new Problem(original);
+        Problem problem = new Problem(original, baseUri);
 
-        assertThat(problem.getType(), is("urn:ietf:params:acme:error:connection"));
+        assertThat(problem.getType(), is(URI.create("urn:ietf:params:acme:error:connection")));
         assertThat(problem.getDetail(), is("connection refused"));
+        assertThat(problem.getInstance(), is(URI.create("https://example.com/documents/error.html")));
         assertThat(problem.asJSON().toString(), is(sameJSONAs(original.toString())));
         assertThat(problem.toString(), is(sameJSONAs(original.toString())));
     }
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
index fd2a8ed2..17d508db 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/connector/DefaultConnectionTest.java
@@ -16,6 +16,7 @@ package org.shredzone.acme4j.connector;
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.*;
 import static org.mockito.Mockito.*;
+import static org.shredzone.acme4j.util.TestUtils.url;
 import static uk.co.datumedge.hamcrest.json.SameJSONAs.sameJSONAs;
 
 import java.io.ByteArrayInputStream;
@@ -425,13 +426,14 @@ public class DefaultConnectionTest {
         when(mockUrlConnection.getHeaderField("Content-Type")).thenReturn("application/problem+json");
         when(mockUrlConnection.getResponseCode()).thenReturn(HttpURLConnection.HTTP_FORBIDDEN);
         when(mockUrlConnection.getErrorStream()).thenReturn(new ByteArrayInputStream(jsonData.getBytes("utf-8")));
+        when(mockUrlConnection.getURL()).thenReturn(url("https://example.com/acme/1"));
 
         try (DefaultConnection conn = new DefaultConnection(mockHttpConnection)) {
             conn.conn = mockUrlConnection;
             conn.accept(HttpURLConnection.HTTP_OK);
             fail("Expected to fail");
         } catch (AcmeServerException ex) {
-            assertThat(ex.getType(), is("urn:ietf:params:acme:error:unauthorized"));
+            assertThat(ex.getType(), is(URI.create("urn:ietf:params:acme:error:unauthorized")));
             assertThat(ex.getMessage(), is("Invalid response: 404"));
         } catch (AcmeException ex) {
             fail("Expected an AcmeServerException");
@@ -440,6 +442,7 @@ public class DefaultConnectionTest {
         verify(mockUrlConnection, atLeastOnce()).getHeaderField("Content-Type");
         verify(mockUrlConnection, atLeastOnce()).getResponseCode();
         verify(mockUrlConnection).getErrorStream();
+        verify(mockUrlConnection).getURL();
         verifyNoMoreInteractions(mockUrlConnection);
     }
 
@@ -452,6 +455,8 @@ public class DefaultConnectionTest {
                 .thenReturn("application/problem+json");
         when(mockUrlConnection.getResponseCode())
                 .thenReturn(HttpURLConnection.HTTP_INTERNAL_ERROR);
+        when(mockUrlConnection.getURL())
+                .thenReturn(url("https://example.com/acme/1"));
 
         try (DefaultConnection conn = new DefaultConnection(mockHttpConnection) {
             @Override
@@ -466,7 +471,7 @@ public class DefaultConnectionTest {
             conn.accept(HttpURLConnection.HTTP_OK);
             fail("Expected to fail");
         } catch (AcmeServerException ex) {
-            assertThat(ex.getType(), is("urn:zombie:error:apocalypse"));
+            assertThat(ex.getType(), is(URI.create("urn:zombie:error:apocalypse")));
             assertThat(ex.getMessage(), is("Zombie apocalypse in progress"));
         } catch (AcmeException ex) {
             fail("Expected an AcmeServerException");
@@ -474,6 +479,7 @@ public class DefaultConnectionTest {
 
         verify(mockUrlConnection).getHeaderField("Content-Type");
         verify(mockUrlConnection, atLeastOnce()).getResponseCode();
+        verify(mockUrlConnection).getURL();
         verifyNoMoreInteractions(mockUrlConnection);
     }
 
@@ -486,6 +492,8 @@ public class DefaultConnectionTest {
                 .thenReturn("application/problem+json");
         when(mockUrlConnection.getResponseCode())
                 .thenReturn(HttpURLConnection.HTTP_INTERNAL_ERROR);
+        when(mockUrlConnection.getURL())
+                .thenReturn(url("https://example.com/acme/1"));
 
         try (DefaultConnection conn = new DefaultConnection(mockHttpConnection) {
             @Override
@@ -504,6 +512,7 @@ public class DefaultConnectionTest {
 
         verify(mockUrlConnection).getHeaderField("Content-Type");
         verify(mockUrlConnection, atLeastOnce()).getResponseCode();
+        verify(mockUrlConnection).getURL();
         verifyNoMoreInteractions(mockUrlConnection);
     }
 
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededExceptionTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededExceptionTest.java
index 8450d6cf..b587d412 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededExceptionTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeRateLimitExceededExceptionTest.java
@@ -15,6 +15,7 @@ package org.shredzone.acme4j.exception;
 
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.assertThat;
+import static org.shredzone.acme4j.util.TestUtils.createProblem;
 
 import java.net.URI;
 import java.time.Duration;
@@ -23,6 +24,7 @@ import java.util.Arrays;
 import java.util.Collection;
 
 import org.junit.Test;
+import org.shredzone.acme4j.Problem;
 
 /**
  * Unit tests for {@link AcmeRateLimitExceededException}.
@@ -34,15 +36,17 @@ public class AcmeRateLimitExceededExceptionTest {
      */
     @Test
     public void testAcmeRateLimitExceededException() {
-        String type = "urn:ietf:params:acme:error:rateLimited";
+        URI type = URI.create("urn:ietf:params:acme:error:rateLimited");
         String detail = "Too many requests per minute";
         Instant retryAfter = Instant.now().plus(Duration.ofMinutes(1));
         Collection<URI> documents = Arrays.asList(
                         URI.create("http://example.com/doc1.html"),
                         URI.create("http://example.com/doc2.html"));
 
+        Problem problem = createProblem(type, detail, null);
+
         AcmeRateLimitExceededException ex
-                = new AcmeRateLimitExceededException(type, detail, retryAfter, documents);
+                = new AcmeRateLimitExceededException(problem, retryAfter, documents);
 
         assertThat(ex.getType(), is(type));
         assertThat(ex.getMessage(), is(detail));
@@ -55,11 +59,13 @@ public class AcmeRateLimitExceededExceptionTest {
      */
     @Test
     public void testNullAcmeRateLimitExceededException() {
-        String type = "urn:ietf:params:acme:error:rateLimited";
+        URI type = URI.create("urn:ietf:params:acme:error:rateLimited");
         String detail = "Too many requests per minute";
 
+        Problem problem = createProblem(type, detail, null);
+
         AcmeRateLimitExceededException ex
-                = new AcmeRateLimitExceededException(type, detail, null, null);
+                = new AcmeRateLimitExceededException(problem, null, null);
 
         assertThat(ex.getType(), is(type));
         assertThat(ex.getMessage(), is(detail));
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredExceptionTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredExceptionTest.java
index bc37791e..db615314 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredExceptionTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/exception/AcmeUserActionRequiredExceptionTest.java
@@ -15,12 +15,14 @@ package org.shredzone.acme4j.exception;
 
 import static org.hamcrest.Matchers.*;
 import static org.junit.Assert.assertThat;
+import static org.shredzone.acme4j.util.TestUtils.createProblem;
 
 import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URL;
 
 import org.junit.Test;
+import org.shredzone.acme4j.Problem;
 
 /**
  * Unit tests for {@link AcmeUserActionRequiredException}.
@@ -32,13 +34,15 @@ public class AcmeUserActionRequiredExceptionTest {
      */
     @Test
     public void testAcmeUserActionRequiredException() throws MalformedURLException {
-        String type = "urn:ietf:params:acme:error:userActionRequired";
+        URI type = URI.create("urn:ietf:params:acme:error:userActionRequired");
         String detail = "Accept new TOS";
         URI tosUri = URI.create("http://example.com/agreement.pdf");
         URL instanceUrl = new URL("http://example.com/howToAgree.html");
 
+        Problem problem = createProblem(type, detail, instanceUrl);
+
         AcmeUserActionRequiredException ex
-            = new AcmeUserActionRequiredException(type, detail, tosUri, instanceUrl);
+            = new AcmeUserActionRequiredException(problem, tosUri);
 
         assertThat(ex.getType(), is(type));
         assertThat(ex.getMessage(), is(detail));
@@ -51,11 +55,13 @@ public class AcmeUserActionRequiredExceptionTest {
      */
     @Test
     public void testNullAcmeUserActionRequiredException() {
-        String type = "urn:ietf:params:acme:error:userActionRequired";
+        URI type = URI.create("urn:ietf:params:acme:error:userActionRequired");
         String detail = "Call our service";
 
+        Problem problem = createProblem(type, detail, null);
+
         AcmeUserActionRequiredException ex
-            = new AcmeUserActionRequiredException(type, detail, null, null);
+            = new AcmeUserActionRequiredException(problem, null);
 
         assertThat(ex.getType(), is(type));
         assertThat(ex.getMessage(), is(detail));
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/util/JSONTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/util/JSONTest.java
index 13838d62..0dfeaf28 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/util/JSONTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/util/JSONTest.java
@@ -26,6 +26,7 @@ import java.io.ObjectInputStream;
 import java.io.ObjectOutputStream;
 import java.net.MalformedURLException;
 import java.net.URI;
+import java.net.URL;
 import java.time.Instant;
 import java.time.LocalDate;
 import java.time.ZoneId;
@@ -45,6 +46,8 @@ import org.shredzone.acme4j.exception.AcmeProtocolException;
  */
 public class JSONTest {
 
+    private static final URL BASE_URL = url("https://example.com/acme/1");
+
     /**
      * Test that an empty {@link JSON} is empty.
      */
@@ -210,10 +213,11 @@ public class JSONTest {
         JSON sub = array.get(3).asObject();
         assertThat(sub.get("test").asString(), is("ok"));
 
-        Problem problem = json.get("problem").asProblem();
+        Problem problem = json.get("problem").asProblem(BASE_URL);
         assertThat(problem, is(notNullValue()));
-        assertThat(problem.getType(), is("urn:ietf:params:acme:error:rateLimited"));
+        assertThat(problem.getType(), is(URI.create("urn:ietf:params:acme:error:rateLimited")));
         assertThat(problem.getDetail(), is("too many requests"));
+        assertThat(problem.getInstance(), is(URI.create("https://example.com/documents/errors.html")));
     }
 
     /**
@@ -231,7 +235,7 @@ public class JSONTest {
         assertThat(json.get("none").asObject(), is(nullValue()));
         assertThat(json.get("none").asStatusOrElse(Status.INVALID), is(Status.INVALID));
         assertThat(json.get("none").asBinary(), is(nullValue()));
-        assertThat(json.get("none").asProblem(), is(nullValue()));
+        assertThat(json.get("none").asProblem(BASE_URL), is(nullValue()));
 
         try {
             json.get("none").asInt();
@@ -308,7 +312,7 @@ public class JSONTest {
         }
 
         try {
-            json.get("text").asProblem();
+            json.get("text").asProblem(BASE_URL);
             fail("no exception was thrown");
         } catch (AcmeProtocolException ex) {
             // expected
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/util/TestUtils.java b/acme4j-client/src/test/java/org/shredzone/acme4j/util/TestUtils.java
index 83f39183..3e1b8e94 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/util/TestUtils.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/util/TestUtils.java
@@ -52,6 +52,7 @@ import org.jose4j.base64url.Base64Url;
 import org.jose4j.json.JsonUtil;
 import org.jose4j.jwk.JsonWebKey;
 import org.jose4j.jwk.JsonWebKey.OutputControlLevel;
+import org.shredzone.acme4j.Problem;
 import org.shredzone.acme4j.Session;
 import org.shredzone.acme4j.provider.AcmeProvider;
 
@@ -268,6 +269,30 @@ public final class TestUtils {
         };
     }
 
+    /**
+     * Creates a {@link Problem} with the given type and details.
+     *
+     * @param type
+     *            Problem type
+     * @param detail
+     *            Problem details
+     * @param instance
+     *            Instance, or {@code null}
+     * @return Created {@link Problem} object
+     */
+    public static Problem createProblem(URI type, String detail, URL instance) {
+        URI baseUri = URI.create("https://example.com/acme/1");
+
+        JSONBuilder jb = new JSONBuilder();
+        jb.put("type", type);
+        jb.put("detail", detail);
+        if (instance != null) {
+            jb.put("instance", instance);
+        }
+
+        return new Problem(jb.toJSON(), baseUri);
+    }
+
     /**
      * Generates a new keypair for unit tests, and return its N, E, KTY and THUMBPRINT
      * parameters to be set in the {@link TestUtils} class.
diff --git a/acme4j-client/src/test/resources/json/datatypes.json b/acme4j-client/src/test/resources/json/datatypes.json
index 72374e6d..90997f90 100644
--- a/acme4j-client/src/test/resources/json/datatypes.json
+++ b/acme4j-client/src/test/resources/json/datatypes.json
@@ -11,6 +11,7 @@
   "binary": "Q2hhaW5zYXc",
   "problem": {
     "type": "urn:ietf:params:acme:error:rateLimited",
-    "detail": "too many requests"
+    "detail": "too many requests",
+    "instance": "/documents/errors.html"
   }
 }
diff --git a/acme4j-client/src/test/resources/json/problem.json b/acme4j-client/src/test/resources/json/problem.json
index 41e78281..f43a53fd 100644
--- a/acme4j-client/src/test/resources/json/problem.json
+++ b/acme4j-client/src/test/resources/json/problem.json
@@ -1,4 +1,5 @@
 {
   "type": "urn:ietf:params:acme:error:connection",
-  "detail": "connection refused"
+  "detail": "connection refused",
+  "instance": "/documents/error.html"
 }