Index: subsystems/win32/win32k/ntuser/window.c
===================================================================
--- subsystems/win32/win32k/ntuser/window.c	(revision 42982)
+++ subsystems/win32/win32k/ntuser/window.c	(working copy)
@@ -1022,6 +1022,13 @@
 //   hWnd = Wnd->hSelf;
 //   hWndNewParent = WndNewParent->hSelf;
 
+    /* Some applications try to set a child as a parent */
+    if (IntIsChildWindow(Wnd, WndNewParent))
+    {
+        SetLastWin32Error( ERROR_INVALID_PARAMETER );
+        return 0;
+    }
+
    /*
     * Windows hides the window first, then shows it again
     * including the WM_SHOWWINDOW messages and all