Basically, if there are no creatures in that index, it will give you an error. I don't know of a way to catch this error other than using error hooks or pCall, so for now, just ignore it; I don't see the point in you worrying about catching errors right now. If it becomes a problem, let me know. It's annoying, but I don't know how to determine which indices contained creatures.
P.S.
Don't slots start at 1?
The manual says it starts at 0.
"The stack index number is selected with the "creatureIndex" parameter and counts off from 0."
