GetWindowRect / hWnd problems

My code is causing assertion errors on several functions that I've used successfully before without problems, such as here I'm using GetWindowRect on a CStatiic variable within a CRecordView class.

Code:

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_AFXWIN_INLINE void CWnd::GetWindowRect(LPRECT lpRect) const
        { ASSERT(::IsWindow(m_hWnd)); ::GetWindowRect(m_hWnd, lpRect); }

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I've used this before in the same place in my code without any problems.

I'm following a tutorial on CodeGuru for creating a property sheet inside a property page, and Sheet.ModifyStyleEx within OnInitDialog on my property page also fails at
ASSERT(hWnd != NULL)
I dont know why this is happenning and any suggestions would be appreciated, thanks.