Problem writing DirectShow Source Filter

[CeeDee666]

I've got problems writing a sourecfilter for directshow. I derrived my class from CSourceStream and implemented the methods like described in the directx docu.
when i test the filter in graphbuilder everything works fine until i press paly. then i get an exception inside FillBuffer(IMediaSample *pms).
I looked at it with the debugger and it seems that i dont bet an IMediaSample from the framework but an IUnknown on which i can't invoke methods like GetPointer().
has anyone an idee where this problem comes from or how to sobe it?

thanks Christian