[UICollectionViewLayout dealloc]中的怪异EXC_BAD_ACCESS

I push a view controller that contains a UICollectionView onto a navigation controller. When I pop it to return to the previous VC, I get an EXC_BAD_ACCESS code=2. I have NSZombies enabled and it stops at the end (the line with the "}") of the dealloc method of my UICollectionViewLayout subclass. This is the stack trace:I push a view controller that contains a UIColl