SubclassWindow of CXTPMDiframeWnd fails

Hi all

I'm trying to get a subclass working that used to work in my non CodeJock app. Now CMainFrame is derived from CXTPMDIFrameWnd, it asserts at the FromHandlePermanent(hWndNew) in CWnd::Attach(). I assume this is because the CodeJock parent class already has some form of subclassing and you can't subclass twice.

The subclassed class simply overrides OnEraseBkgnd and OnSize and paints text to the MDI frame background as described in many tutorials and examples on the net.
In my CMainFrame::OnCreate() I call:

//subclass to allow erasebkgnd to repaint colour of mainframe
CWnd* pChildWnd = GetWindow(GW_CHILD);
if (pChildWnd)
{
    if (!m_wndMDIClient.SubclassWindow (pChildWnd -> m_hWnd)) {
        TRACE ("Failed to subclass MDI client window\n");
        return (-1);
    }
}

Any thoughts on how I can re-implement this functionality.
Thanks
Al.

No, I derive my CMainFrame from CXTPMDIFrameWnd. The document type we use is a fixed size dialog so doesn't really work with a tabbed client format.

Thanks
Al.