Index: /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 13066)
+++ /trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 13067)
@@ -450,6 +450,11 @@
                 }
             }
-            if (fixed && !relation.isNew())
-                commands.add(new ChangeCommand(relation, r2));
+            if (fixed && !relation.isNew()) {
+                DataSet ds = relation.getDataSet();
+                if (ds == null) {
+                    ds = MainApplication.getLayerManager().getEditDataSet();
+                }
+                commands.add(new ChangeCommand(ds, relation, r2));
+            }
         }