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 5e1e8f9e..d8d87e2a 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
@@ -213,8 +213,8 @@ public class DefaultConnection implements Connection {
         assertConnectionIsOpen();
 
         String contentType = conn.getHeaderField(CONTENT_TYPE_HEADER);
-        if (!("application/json".equals(contentType)
-                    || "application/problem+json".equals(contentType))) {
+        if (contentType == null || !(contentType.contains("application/json")
+                || contentType.contains("application/problem+json"))) {
             throw new AcmeProtocolException("Unexpected content type: " + contentType);
         }