DestroyWindow ASSERT problem....strange, strange!

Hi

I have derived a dialog-class in a DLL (shared MFC)
I can construct an object of this class in my app, and when overloading

DoModal() and adding an AFX_MANAGE_STATE(AfxGetStaticModuleState( ));
call, I can see the CMyDialog dialog.

But here is the issue, I have a timer set for a certain time, it posts
a message and the dialog has to be killed - so I use CDialog::OnCancel.

Then DoModal calls DestroyWindow() -
Here a copy of the m_hWnd is made in hWndOrig.
#ifdef _DEBUG
        hWndOrig = m_hWnd;
#endif

Then m_hWnd gets set to Null by ::DestroyWndow (which calls
OnNcDestroy).
But later in DestroyWindow itself, there is a check:
#ifdef _DEBUG
    ASSERT(m_hWnd == hWndOrig);
#endif

And the program asserts here, which is expected as the ::DestroyWindow
call before this set the m_hWnd to NULL......this is strange!

Please advise, I saw similar queries posted before, but no answers..I
have pasted the Wincore.cpp DestroyWindow() code below for reference...

Thanks!

    if (m_hWnd != NULL)
    {
        pMap = afxMapHWND();
        ENSURE(pMap != NULL);
        pWnd = (CWnd*)pMap->LookupPermanent(m_hWnd);
#ifdef _DEBUG
        hWndOrig = m_hWnd;
#endif
    }

#ifdef _AFX_NO_OCC_SUPPORT
    if (m_hWnd != NULL)
        bResult = ::DestroyWindow(m_hWnd);
#else //_AFX_NO_OCC_SUPPORT
    if ((m_hWnd != NULL) || (m_pCtrlSite != NULL))
    {
        if (m_pCtrlSite == NULL)
            bResult = ::DestroyWindow(m_hWnd);
        else
            bResult = m_pCtrlSite->DestroyControl();
    }
#endif //_AFX_NO_OCC_SUPPORT

    if (hWndOrig != NULL)
    {
        // Note that 'this' may have been deleted at this point,
        // (but only if pWnd != NULL)
        if (pWnd != NULL)
        {
            // Should have been detached by OnNcDestroy
#ifdef _DEBUG
            ASSERT(pMap->LookupPermanent(hWndOrig) == NULL);
#endif
        }
        else
        {
#ifdef _DEBUG
            ASSERT(m_hWnd == hWndOrig);
#endif
        // Detach after DestroyWindow called just in case
            Detach();
        }