UnitGUID issue in flex mode?

[zork]

Someone posted this error in the comments on my tooltip mod

Message: Interface\AddOns\rTooltip\core.lua:128: Usage: UnitGUID("unit") Time: 09/14/13 18:55:34 Count: 8 Stack: [C]: in function `UnitGUID' Interface\AddOns\rTooltip\core.lua:128: in function <Interface\AddOns\rTooltip\core.lua:126> [C]: ? Locals: (*temporary) = nil

What I'm doing is the following:

local unit = select(2, self:GetUnit()) or (GetMouseFocus() and GetMouseFocus():GetAttribute("unit")) or (UnitExists("mouseover") and "mouseover")
local guid = UnitGUID(unit) or nil
if not guid then return end

Full code: http://code.google.com/p/rothui/sour...p/core.lua#125

I tested UnitGUID(unitID). When entering an invalid unitID it just returns nil.
What could make the function fail? It works flawlessly in any case I know. No issues with cross-realms.

I have not tested Flex raids yet. Am I missing something?

*edit* ------------------------
Ok...I think I forgot to catch nil and type. So going to add:

if not unit or (unit and type(unit) ~= "string") then return end

This should do it.