14w31a crash

An iron golem hit a zombie and the game crashed

What I expected to happen was...:
The iron golem would kill the zombie

What actually happened was...:
The game crashed

Steps to Reproduce:
1. Spawn an iron golem, name it Guard (with a nametag), tie it up (with a lead) to a fence and spawn a zombie.
2. Let the Guard iron golem hit the zombie
3. The game will crash