afxwin2.inl line 265

I have an MDI app, and from a dialogbar, I open a child, one by one ... If something goes wrong, I display an modal dialog, by the cause ... the issue is that after I close this dialog modal, I get an assert error:

"Debug assertion failed !"

File Afxwin2.inl, line 265 ... and there, I get:

_AFXWIN_INLINE CWnd* CWnd::GetParent() const
	{ ASSERT(::IsWindow(m_hWnd)); return CWnd::FromHandle(::GetParent(m_hWnd)); }

I had tried a lot of posible causes ... have you met by this assertion ? What is the real cause ? Can you help me ? Thank you.