The robloxdev site states this as the coroutine.resume signature:
bool, Variant coroutine.resume ( thread co, number val1, number ... )
However, the parameter list is incorrect. val1 and the subsequent varargs are not necessarily “numbers” but can be of any type. It appears that “Variant” is the word used for a value of any type.
This tripped me up, since I was trying to double-check what the method returns, and then saw that it listed number for the parameter types, and had to do some external Googling to figure out that it was incorrect.
While this is small, it can easily trip up newer developers.