The IterateOverReachableObjects functions is specified so that if the heap root callback is NULL then heap roots are not reported. Also if the stack ref callback is NULL then stack references are not reported. This is interpreted in the implementation as "follows" which means that IterateOverReachableObjects(NULL, NULL, object_ref, user_data) is no-op as there are initial objects (roots) pushed to the visit stack. This should be fixed.