Index: /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 6568)
+++ /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 6569)
@@ -93,10 +93,20 @@
         }
 
+        final Relation multipolygonRelation = getSelectedMultipolygonRelation(selectedRelations);
+        if (multipolygonRelation != null && (multipolygonRelation.isIncomplete() || multipolygonRelation.hasIncompleteMembers())) {
+            new Notification(
+                    tr("Cannot update multipolygon since relation is incomplete."))
+                    .setIcon(JOptionPane.WARNING_MESSAGE)
+                    .setDuration(Notification.TIME_DEFAULT)
+                    .show();
+            return;
+        }
+
         final Pair<SequenceCommand, Relation> commandAndRelation = createMultipolygonCommand(selectedWays, selectedRelations);
+        if (commandAndRelation == null) {
+            return;
+        }
         final Command command = commandAndRelation.a;
         final Relation relation = commandAndRelation.b;
-        if (command == null) {
-            return;
-        }
         Main.main.undoRedo.add(command);