ONOS-4721 fix

Change-Id: Idb0ba949293451488b350c7cdcfc50094a3c9abc
diff --git a/core/common/src/main/java/org/onosproject/codec/impl/GroupCodec.java b/core/common/src/main/java/org/onosproject/codec/impl/GroupCodec.java
index f6e3e9d..0afd71c 100644
--- a/core/common/src/main/java/org/onosproject/codec/impl/GroupCodec.java
+++ b/core/common/src/main/java/org/onosproject/codec/impl/GroupCodec.java
@@ -146,8 +146,7 @@
                 groupType = Group.Type.FAILOVER;
                 break;
             default:
-                log.warn("The requested type {} is not defined for group.", type);
-                return null;
+                nullIsIllegal(groupType, "The requested group type " + type + " is not valid");
         }
 
         // parse group buckets