Was running my OS on SwitchCraft, and I encountered this error.
Was also told by Lemmmy that this error may have been caused by a recent LuaJ rewrite.

The error gets thrown in the section of my OS that handles the coroutines, which means that it's system level.
Attached below is an installer of the OS that I encountered the error on.
installer.zip

Minecraft version 1.12.2, Forge 14.23.4.2759

Update: My code does not work in CCEmuX either. Latest version

Yeah, CCEmuX is using CC:T so that makes sense.

I've made a workaround in my OS, so please keep your current files and do not update them if you still need them.