Problem with compatibilty with Leopard released ver.

Hi all

I have been facing a tricky problem. I have placed a transparent overlay window (using NSWindow class) over an application icon in the "Dock". This is working fine in the MAC OS X 10.4.11 however when I have tried running the same application in the 10.5 i.e. MAC OS X 10.5 (Leopard), the same window fails. This is happening at the
[super initWithContentRect:contentRect styleMask:NSBorderlessWindowMask backing:NSBackingStoreBuffered defer:NO] call. I am not able to get what is going wrong. This class is inherited from the NSWindow class.
The error that i am getting is:
Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Error (1002) creating CGSWindow'

Thanks in advance for any help.