Hi I was just looking for the same bug and I can cross this issue with the explanation from the terrafirmacraft team.
Could you look at it?
Kittychanley
http://terrafirmacraft.com/f/topic/9581-problem-with-custom-npcs/
and also here
http://terrafirmacraft.com/f/topic/9137-solved-custom-npc-tfc-damagearmor-calculation-crash/
Can you look at the code if it's true because now it ended up in a dead end. And possibly backport the fix to 1.7.10? Both mods are awesome but they can't be used together just for this calculation crash.
Thanks