Index: trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 17217)
+++ trunk/src/org/openstreetmap/josm/actions/CreateMultipolygonAction.java	(revision 17219)
@@ -304,4 +304,5 @@
             showErrors(mpTest.getErrors());
         }
+        calculated.setMembers(null); // see #19885
         return null; //could not make multipolygon.
     }