Internal Exeption: java.lang.ClassCastExeption: bit cannot be cast to fm

What I expected to happen was...:
That my Singleplayer World would load properly

What actually happened was...:
I got this error:
http://imgur.com/4wIubhG
(Follow link)

Steps to Reproduce:
1. Create a Single player world, any gamemode, normal generation, cheats. Have not tested under different circumstances, have altered some unlisted circumstances.
2. Load the level.
3. Game crashes with above error and when I press "Back to Title Screen", It brings me to the Multiplayer list.