Thank you for the report! It seems my logic for determining what vehicles have ammo-consuming guns is flawed. The lasertank is using lasertanks-ammo-[1-3], but as there are no recipes for crafting these ammo types, the lasertank's gun is ignored, so Autodrive assumes the lasertank has 0 guns. This results in trying to set the gun index to an illegal value.
Now I've just got to figure out how to fix this. :-)