Failed to load mods: fall-of-promethea/prototypes/entities.lua:78: attempt to index field '?' (a nil value)
stack traceback:
fall-of-promethea/prototypes/entities.lua:78: in main chunk
[C]: in function 'require'
fall-of-promethea/data.lua:38: in main chunk

Please post your mod list, where you saw this error, instructions to reproduce the error, OS, FoP mod version, and any details you think might be helpful. This portion of code deals with Promethium Thrusters, but that could be misleading.

Possible Alatar's Rocketry https://mods.factorio.com/mod/alatar-rocketry
Disabling this mod and re-enabling your mod.

Aaaannd found the culprit. I disabled Alatar's Rocketry and your mod loads. It used to load along with AR before, but you updated recently and now it does not load together. I assume you added rockets that his also adds?

Thanks, I took a look and I don't see a way to make the mods compatible. I might put some more time in later on, but for now I'll note it as incompatible.

I took another look at this. That mod removes the default prototype for the thruster, which mine uses. I'm not going to be able to fix this one.