I've seen this happen if the AI decides to set the max Stomach to 0.
You can trivially repro this if you save a world, edit max stomach size to 0 in the json, then load that world and skip customize.
That said, if anyone wanted to somehow involve a dullahan in their world, you now have a way to do that and even get the game to render your free-floating head. A little dividing by 0 never hurt anyone...
Apparently this can also happen if max Fatness is changed to 0.
So repro: make either Max Stomach=0 or Max Fatness=0