Yes, it was a bit of a simplification on the real iterator that was causing the problem, but I forgot that I hadn't implement post increment correctly. At the time I didn't see how it would get me into trouble, now I do :) My input iterator is a little tricky to create because I don't know how many times it can be incremented before reaching the end. The end() iterator simply holds a NULL pointer, incrementing it does nothing. When the begin() iterator is incremented enough I set it's pointer to NULL and it becomes equal to end(). This causes a lot of non-standard issues ( like end() == end()+1 ). I'm hoping that particular one doesn't cause a problem for me. Again it seems the trouble is with me rather than Boost::Python. Thanks for setting me straight, you saved me a lot of debugging time. -Dan