It was his code the created the null reference. He converted a null pointer into a reference. But doesn't matter: when presented with the evidence of a null reference, it would have been straightforward to diagnose the problem. And after repeated blustering and denials from him, that's what I had to do to get the bug fixed. We do a disservice if we tell people that references (in either language) cannot be null. They can be null, and that would be a bug in some other code - so follow the trail to find where.