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 26d755f2..9cc19a04 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
@@ -232,7 +232,8 @@ public class DefaultConnection implements Connection {
                 return match.getAsInt();
             }
 
-            if (!"application/problem+json".equals(conn.getHeaderField(CONTENT_TYPE_HEADER))) {
+            String contentType = AcmeUtils.getContentType(conn.getHeaderField(CONTENT_TYPE_HEADER));
+            if (!"application/problem+json".equals(contentType)) {
                 throw new AcmeException("HTTP " + rc + ": " + conn.getResponseMessage());
             }
 
@@ -249,7 +250,7 @@ public class DefaultConnection implements Connection {
     public JSON readJsonResponse() throws AcmeException {
         assertConnectionIsOpen();
 
-        String contentType = conn.getHeaderField(CONTENT_TYPE_HEADER);
+        String contentType = AcmeUtils.getContentType(conn.getHeaderField(CONTENT_TYPE_HEADER));
         if (!("application/json".equals(contentType)
                     || "application/problem+json".equals(contentType))) {
             throw new AcmeProtocolException("Unexpected content type: " + contentType);
@@ -275,7 +276,7 @@ public class DefaultConnection implements Connection {
     public List<X509Certificate> readCertificates() throws AcmeException {
         assertConnectionIsOpen();
 
-        String contentType = conn.getHeaderField(CONTENT_TYPE_HEADER);
+        String contentType = AcmeUtils.getContentType(conn.getHeaderField(CONTENT_TYPE_HEADER));
         if (!("application/pem-certificate-chain".equals(contentType))) {
             throw new AcmeProtocolException("Unexpected content type: " + contentType);
         }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
index a7d651a6..2fd75083 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
@@ -58,6 +58,9 @@ public final class AcmeUtils {
     private static final Pattern TZ_PATTERN = Pattern.compile(
                 "([+-])(\\d{2}):?(\\d{2})$");
 
+    private final static Pattern CONTENT_TYPE_PATTERN = Pattern.compile(
+                "([^;]+)(?:;.*?charset=(\"?)([a-z0-9_-]+)(\\2))?.*", Pattern.CASE_INSENSITIVE);
+
     private static final Base64.Encoder PEM_ENCODER = Base64.getMimeEncoder(64, "\n".getBytes());
 
     /**
@@ -326,4 +329,27 @@ public final class AcmeUtils {
         }
     }
 
+    /**
+     * Extracts the content type of a Content-Type header.
+     *
+     * @param header
+     *            Content-Type header
+     * @return Content-Type, or {@code null} if the header was invalid or empty
+     * @throws AcmeProtocolException
+     *             if the Content-Type header contains a different charset than "utf-8".
+     */
+    public static String getContentType(String header) {
+        if (header != null) {
+            Matcher m = CONTENT_TYPE_PATTERN.matcher(header);
+            if (m.matches()) {
+                String charset = m.group(3);
+                if (charset != null && !"utf-8".equalsIgnoreCase(charset)) {
+                    throw new AcmeProtocolException("Unsupported charset " + charset);
+                }
+                return m.group(1).trim().toLowerCase();
+            }
+        }
+        return null;
+    }
+
 }
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
index 4ac316d1..50cb6146 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
@@ -43,6 +43,7 @@ import org.hamcrest.Description;
 import org.jose4j.jwk.PublicJsonWebKey;
 import org.junit.BeforeClass;
 import org.junit.Test;
+import org.shredzone.acme4j.exception.AcmeProtocolException;
 
 /**
  * Unit tests for {@link AcmeUtils}.
@@ -299,6 +300,35 @@ public class AcmeUtilsTest {
         assertThat(AcmeUtils.toURL(null), is(nullValue()));
     }
 
+    /**
+     * Test {@link AcmeUtils#getContentType(String)}.
+     */
+    @Test
+    public void testGetContentType() {
+        assertThat(AcmeUtils.getContentType(null), is(nullValue()));
+        assertThat(AcmeUtils.getContentType("application/json"),
+                        is("application/json"));
+        assertThat(AcmeUtils.getContentType("Application/Problem+JSON"),
+                        is("application/problem+json"));
+        assertThat(AcmeUtils.getContentType("application/json; charset=utf-8"),
+                        is("application/json"));
+        assertThat(AcmeUtils.getContentType("application/json; charset=utf-8 (Plain text)"),
+                        is("application/json"));
+        assertThat(AcmeUtils.getContentType("application/json; charset=\"utf-8\""),
+                        is("application/json"));
+        assertThat(AcmeUtils.getContentType("application/json; charset=\"UTF-8\"; foo=4"),
+                        is("application/json"));
+        assertThat(AcmeUtils.getContentType(" application/json ;foo=4"),
+                        is("application/json"));
+
+        try {
+            AcmeUtils.getContentType("application/json; charset=\"iso-8859-1\"");
+            fail("Accepted bad charset");
+        } catch (AcmeProtocolException ex) {
+            // expected
+        }
+    }
+
     /**
      * Matches the given time.
      */