diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
index ce130ad7..dc6c0480 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Session.java
@@ -27,9 +27,9 @@ import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.StreamSupport;
 
 import org.shredzone.acme4j.challenge.Challenge;
-import org.shredzone.acme4j.challenge.TokenChallenge;
 import org.shredzone.acme4j.connector.Resource;
 import org.shredzone.acme4j.exception.AcmeException;
+import org.shredzone.acme4j.exception.AcmeProtocolException;
 import org.shredzone.acme4j.provider.AcmeProvider;
 import org.shredzone.acme4j.toolbox.JSON;
 
@@ -174,15 +174,9 @@ public class Session {
      * @return {@link Challenge} instance
      */
     public Challenge createChallenge(JSON data) {
-        Objects.requireNonNull(data, "data");
-
         Challenge challenge = provider().createChallenge(this, data);
         if (challenge == null) {
-            if (data.contains("token")) {
-                challenge = new TokenChallenge(this, data);
-            } else {
-                challenge = new Challenge(this, data);
-            }
+            throw new AcmeProtocolException("Could not create challenge for: " + data);
         }
         return challenge;
     }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
index 7ebb5be0..e61b7ddf 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/provider/AbstractAcmeProvider.java
@@ -25,6 +25,7 @@ import org.shredzone.acme4j.challenge.Challenge;
 import org.shredzone.acme4j.challenge.Dns01Challenge;
 import org.shredzone.acme4j.challenge.Http01Challenge;
 import org.shredzone.acme4j.challenge.TlsSni02Challenge;
+import org.shredzone.acme4j.challenge.TokenChallenge;
 import org.shredzone.acme4j.connector.Connection;
 import org.shredzone.acme4j.connector.DefaultConnection;
 import org.shredzone.acme4j.connector.HttpConnector;
@@ -72,6 +73,9 @@ public abstract class AbstractAcmeProvider implements AcmeProvider {
     /**
      * {@inheritDoc}
      * <p>
+     * This implementation handles the standard challenge types. For unknown types,
+     * generic {@link Challenge} or {@link TokenChallenge} instances are created.
+     * <p>
      * Custom provider implementations may override this method to provide challenges that
      * are unique to the provider.
      */
@@ -83,11 +87,15 @@ public abstract class AbstractAcmeProvider implements AcmeProvider {
         String type = data.get("type").required().asString();
 
         BiFunction<Session, JSON, Challenge> constructor = CHALLENGES.get(type);
-        if (constructor == null) {
-            return null;
+        if (constructor != null) {
+            return constructor.apply(session, data);
         }
 
-        return constructor.apply(session, data);
+        if (data.contains("token")) {
+            return new TokenChallenge(session, data);
+        } else {
+            return new Challenge(session, data);
+        }
     }
 
     /**
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/provider/AbstractAcmeProviderTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/provider/AbstractAcmeProviderTest.java
index 125ccdff..a197a03c 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/provider/AbstractAcmeProviderTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/provider/AbstractAcmeProviderTest.java
@@ -155,7 +155,8 @@ public class AbstractAcmeProviderTest {
                     .put("url", "https://example.com/some/challenge")
                     .toJSON();
         Challenge c6 = provider.createChallenge(session, json6);
-        assertThat(c6, is(nullValue()));
+        assertThat(c6, not(nullValue()));
+        assertThat(c6, instanceOf(Challenge.class));
 
         try {
             JSON json7 = new JSONBuilder()