--- a/WindowGroup.st	Thu Aug 22 22:50:00 2013 +0200
+++ b/WindowGroup.st	Thu Aug 22 23:08:24 2013 +0200
@@ -584,6 +584,7 @@
      Exposeevents for that windowGroup will be handled by this group
      as well."
 
+    self assert:(aWindowGroup ~~ self).
     previousGroup := aWindowGroup
 
     "Created: 20.8.1997 / 17:57:35 / cg"
@@ -1007,7 +1008,10 @@
 
     "set previousGroup to the main (non-modal) group"
 
-    previousGroup isNil ifTrue:[previousGroup := mainGroup].
+    previousGroup isNil ifTrue:[
+        self assert:(mainGroup ~~ self).
+        previousGroup := mainGroup
+    ].
     prevGroup := previousGroup.
     prevGroup notNil ifTrue:[
         "/ remember current grab, in case I am opened by
@@ -2573,6 +2577,7 @@
      set the windowgroup that started this group (for modal groups only).
      This is not a public interface."
 
+    self assert:(aGroup ~~ self).
     previousGroup := aGroup
 
     "Modified: 3.9.1995 / 14:55:40 / claus"
@@ -2769,11 +2774,11 @@
 !WindowGroup class methodsFor:'documentation'!
 
 version
-    ^ '$Header: /cvs/stx/stx/libview/WindowGroup.st,v 1.319 2013-07-13 20:49:51 cg Exp $'
+    ^ '$Header: /cvs/stx/stx/libview/WindowGroup.st,v 1.320 2013-08-22 21:08:24 cg Exp $'
 !
 
 version_CVS
-    ^ '$Header: /cvs/stx/stx/libview/WindowGroup.st,v 1.319 2013-07-13 20:49:51 cg Exp $'
+    ^ '$Header: /cvs/stx/stx/libview/WindowGroup.st,v 1.320 2013-08-22 21:08:24 cg Exp $'
 ! !