Index: trunk/src/org/openstreetmap/josm/actions/JoinAreasAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/JoinAreasAction.java	(revision 15623)
+++ trunk/src/org/openstreetmap/josm/actions/JoinAreasAction.java	(revision 15627)
@@ -1346,5 +1346,7 @@
 
         Pair<Way, Command> result = CombineWayAction.combineWaysWorker(actionWays);
-
+        if (result == null) {
+            throw new JosmRuntimeException("Join areas internal error.");
+        }
         commitCommand(result.b);
         cmdsCount++;