EDIT: Actually, the error seems to have disappeared (at startup, at least?) with the new update.

I'm getting this when starting up the game (it appears about the same time as the "This is early access" notification). I suspect it's appearing so early because my character actually has a 9mm box in my inventory.

It also appears randomly a few times throughout gameplay too (thankfully not too often, though it did appear once during a fight with a zombie).