diff --git a/java_gen/templates/of_factory_class.java b/java_gen/templates/of_factory_class.java
index deb6e53..ef26ca0 100644
--- a/java_gen/templates/of_factory_class.java
+++ b/java_gen/templates/of_factory_class.java
@@ -121,7 +121,7 @@
                 return (OFOxm<F>)((Object)${method_name}((${type_name})((Object)value)));
             //:: #endfor
             default:
-                return null;
+                throw new IllegalArgumentException("No OXM known for match field " + field);
         }
     }
 
@@ -140,7 +140,7 @@
                 return (OFOxm<F>)((Object)${method_name}((${type_name})((Object)value), (${type_name})((Object)mask)));
             //:: #endfor
             default:
-                return null;
+                throw new IllegalArgumentException("No OXM known for match field " + field);
         }
     }