PROBLEM SOLVED
This has popped up a few times before (the question of what causes it) so if the mods feel so inclined a sticky thread might be useful.
I only got this error when I was working on a 190a5 skin. I wasn't sure what caused it. The problem went away *instantly* when I included an object.txt file (my 190a5.txt file in the skin directory I was working on). I'm not sure why it started giving me the invalid model number, but it was related to the skin I was building. This has fixed the problem on my end.