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 2334c3d6..4e412708 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
@@ -26,6 +26,9 @@ import org.shredzone.acme4j.Problem;
 
 /**
  * An exception that is thrown when the user is required to take action as indicated.
+ * <p>
+ * Usually this exception is thrown when the terms of service have changed, and the CA
+ * requires an agreement to the new terms before proceeding.
  */
 @ParametersAreNonnullByDefault
 @Immutable
@@ -57,15 +60,14 @@ public class AcmeUserActionRequiredException extends AcmeServerException {
     }
 
     /**
-     * Returns the {@link URL} of a document indicating the action required by the user,
-     * or {@code null} if the server did not provide such a link.
+     * Returns the {@link URL} of a document that gives instructions on the actions to
+     * be taken by a human.
      */
-    @CheckForNull
     public URL getInstance() {
         URI instance = getProblem().getInstance();
 
         if (instance == null) {
-            return null;
+            throw new AcmeProtocolException("Instance URL required, but missing.");
         }
 
         try {
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 8dc398a5..4d0415cb 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
@@ -54,11 +54,12 @@ public class AcmeUserActionRequiredExceptionTest {
      * Test that optional parameters are null-safe.
      */
     @Test
-    public void testNullAcmeUserActionRequiredException() {
+    public void testNullAcmeUserActionRequiredException() throws MalformedURLException {
         URI type = URI.create("urn:ietf:params:acme:error:userActionRequired");
         String detail = "Call our service";
+        URL instanceUrl = new URL("http://example.com/howToContactUs.html");
 
-        Problem problem = createProblem(type, detail, null);
+        Problem problem = createProblem(type, detail, instanceUrl);
 
         AcmeUserActionRequiredException ex
             = new AcmeUserActionRequiredException(problem, null);
@@ -66,7 +67,7 @@ public class AcmeUserActionRequiredExceptionTest {
         assertThat(ex.getType(), is(type));
         assertThat(ex.getMessage(), is(detail));
         assertThat(ex.getTermsOfServiceUri(), nullValue());
-        assertThat(ex.getInstance(), nullValue());
+        assertThat(ex.getInstance(), is(instanceUrl));
     }
 
 }
diff --git a/src/site/markdown/usage/account.md b/src/site/markdown/usage/account.md
index deb12510..120461df 100644
--- a/src/site/markdown/usage/account.md
+++ b/src/site/markdown/usage/account.md
@@ -69,6 +69,8 @@ Now you have a key pair and the account's location URL. This is all you need for
 Even if it is tempting to do so, you should not invoke `agreeToTermsOfService()` automatically, but let the user confirm the terms of service first. To get a link to the current terms of service, you can invoke `session.getMetadata().getTermsOfService()`.
 </div>
 
+If the CA changes the terms of service and requires an explicit agreement to the new terms, an `AcmeUserActionRequiredException` is thrown. Its `getInstance()` method returns the URL of a document that gives instructions about how to agree to the new terms of service. There is no way to automatize this process.
+
 ## Find out Your Account's Location URL
 
 If you only have your account's `KeyPair`, you can use the `AccountBuilder` to find out the location `URL` of your account.