Riding horse crashes client

Put the summary of the bug you're having here

What I expected to happen was...:
Right-clicking horse to break and tame it.

What actually happened was...:
When right-clicking horse, crashed client. Had to NBT edit the player.dat file to re-connect to server.

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...