diff --git a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
index 2e7ff5e..4fd9b1c 100644
--- a/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
+++ b/java_gen/pre-written/src/main/java/org/projectfloodlight/openflow/types/IPv6AddressWithMask.java
@@ -90,12 +90,15 @@
 
     @Override
     public boolean contains(IPAddress<?> ip) {
-        if (string == null) {
-            throw new NullPointerException("String must not be null");
+        if (ip == null) {
+            throw new NullPointerException("ip must not be null");
         }
-        
-        IPv6Address ipv6 = (IPv6Address) ip;
-        return this.matches(ipv6);
-    }
 
+        if(ip.getIpVersion() == IPVersion.IPv6) {
+            IPv6Address ipv6 = (IPv6Address) ip;
+            return this.matches(ipv6);
+        }
+
+        return false;
+    }
 }