This has been an issue for a while, but I haven't had the time to fix it yet. I looked at it today, and it seems to be a little more complicated than the arm code being broken. It looks like the bug is somewhere else, but the arm is where it breaks. I will look further into it, I assume it's maybe an item id somewhere being misspelled or something.
If you want to provide a world download, I can use it to track down the bug easier.