Whenever a zombie kills an entity that's not a building, the game freezes but the popup never happens so it's hard to know exactly what's causing it because it freezes but never outputs an error in the log

So if they destroy a tree or a rock this happens? Yeah ok that makes sense, I'll get that fixed when I get a moment

You'll have to give me an example actually, what entity are you talking about?

Sorry I was so tired when I wrote that, it wasn't very helpful. I was finally able to get an error. It finally popped up when they took out a regular gun turret in wave defense. Doesn't happen if there is no iron or copper in the recipe. Walls and basic furnaces were fine. Rocks and trees also fine. Turrets, assembly machine, etc triggered it

Error:

Error while running event ZombieHorde::on_entity_died (ID 4)
position not specified.
stack traceback:
[C]: in function 'create_entity'
ZombieHorde/libs/zombies.lua:87: in function 'spawn_based_on_ingredient'
ZombieHorde/libs/zombies.lua:48: in function 'on_eat_building'
ZombieHorde/control.lua:63: in function
<ZombieHorde/control.lua:62>

Right, Wave defense. Well there is a mod option to disable zombies spawning on destroyed buildings, you can turn that off for now. I'll see if I can get it fixed though.

Finally had some time to look into it. Was because wave defense removes the default surface which the logic relied on.
Updated so it should work with any mods that change the surface now.