Strange error? "bad argument #1 to 'pairs' (table expected, got number)"

elements = {
	ElemA = 1,
	ElemB = 2,
	ElemC = 3,
	Wood = 4,
	Metal = 5,
}
function elements:bynum(self, num)
	for i, v in pairs(self) do
		if num == v then return i end
	end
end

bad argument #1 to 'pairs' (table expected, got number) stack traceback: