Minecraft 1.6.4 crash and spawn chunk deleted

I loaded my (epic) mob arena map created in snapshot 14w11b in 1.6.4 for testing, in every try to load it Minecraft crashes. I opened it in 14w11b and the spawn chunk was simply gone.

The mob arena had command blocks and a mob spawner with multiple spawning options.

Please fix, I must rebuild the whole spawn again :3

*note: this bug effects 1.6.4. I wrote 14w11b in "affected versions" because 1.6 - 1.6.4 wasn't an option.