I am playing the 1.7.10 release of Crash Landing HQM. Every time I kill a compressed zombie, the game crashes back to the launcher with the following crash: http://pastebin.com/b0scbYHx I have already updated Ex Compressum to the most recent release for 1.7.10, and the crash still occurs.

I have also cross posted this on your issue tracker on Curse: https://minecraft.curseforge.com/projects/ex-compressum/issues/5

After encountering a compressed Creeper, it would appear that this issue is affecting ALL compressed mobs.

As a test thinking it was the fact I was using a tinkers tool to kill them with, I killed one with a piece of cobble, and still crashed http://pastebin.com/xDKjZzLf

I have also removed zombies from the compressed spawn list, in the hopes that it will at least prevent this from happening due to the compressed mobs.

Looks like some compatibility issue, might be a coremod or something replacing the vanilla entities and not implementing the .getEntityData() method correctly. I suggest just disabling them in the config since this seems to be an isolated case and I'd rather not dig up my 1.7.10 workspace to "fix" it at this point.