Thats... unique. which mods were you playing with? If you can give me a list I can try and reproduce this. For now though (and just in case) I added a check to make sure the value being tested is a number; though factorio wiki explicitly states it must be an int...

though factorio wiki explicitly states it must be an int...

Other mods can set it to strings...

and I think I also know which mod causes this crash. Going to hit up the mod author about the second time with strings and ints confussions.

Well, the update should work with strings (it will just assume default factorio behavior for those recipes without crashing), so you can try it out. Overall though I kind of come from a c++ background where there is a bit less of a chance of someone setting a string value to an integer variable. Never realized that such a thing was possible in lua. I mean - I knew you can set variables to whatever you felt like, but I thought factorio documentation could be trusted :)

I'll mark this down as solved then.