Index: /trunk/src/org/openstreetmap/josm/actions/relation/DuplicateRelationAction.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/actions/relation/DuplicateRelationAction.java	(revision 14240)
+++ /trunk/src/org/openstreetmap/josm/actions/relation/DuplicateRelationAction.java	(revision 14241)
@@ -52,10 +52,13 @@
     }
 
+    private static boolean isEditableRelation(IRelation<?> r) {
+        return r instanceof Relation && r.getDataSet() != null && !r.getDataSet().isLocked();
+    }
+
     @Override
     protected void updateEnabledState() {
         // only one selected relation can be edited
         setEnabled(relations.size() == 1
-                && relations.iterator().next() instanceof Relation
-                && !relations.iterator().next().getDataSet().isLocked());
+                && isEditableRelation(relations.iterator().next()));
     }
 }