Index: win32ss/user/ntuser/windc.c
===================================================================
--- win32ss/user/ntuser/windc.c	(revision 64034)
+++ win32ss/user/ntuser/windc.c	(working copy)
@@ -255,8 +255,11 @@
       if(Dce->hrgnClip != NULL)
       {
          PREGION RgnClip = REGION_LockRgn(Dce->hrgnClip);
-         IntGdiCombineRgn(RgnVisible, RgnVisible, RgnClip, RGN_AND);
-         REGION_UnlockRgn(RgnClip);
+         if(RgnClip != NULL) 
+         {
+             IntGdiCombineRgn(RgnVisible, RgnVisible, RgnClip, RGN_AND);
+             REGION_UnlockRgn(RgnClip);
+         }
       }
       else
       {
