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implementing NonDelegatingQueryInterface

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I am trying to implement seeking on a directshow object, but I think this might be a general COM question. Here is my implementation:
STDMETHODIMP CDBGVideoSourcePin::NonDelegatingQueryInterface(REFIID riid, void **ppv)
	if( riid == IID_IMediaSeeking ) 
		return CSourceSeeking::NonDelegatingQueryInterface( riid, ppv );
	return CSourceStream::NonDelegatingQueryInterface(riid, ppv);
If I run this, it crashes when CSourceStream calls FillBuffer (pure virtual function call). If I comment out the two lines about MediaSeeking in the above function, it doesn't crash. What is going on here? My only guess is that some of the CSourceStream query interface calls are returning interfaces in the CSourceSeeking object.

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