Parameter turns nil

Question

Okay, I'm currently experiencing the following error when ingame:

...Addons\WatcherEx\GeminiSpellBook\GeminiSpellBook.lua:91: attempt to index local 'spell' (a nil value)
stack trace:
    ...Addons\WatcherEx\GeminiSpellBook\GeminiSpellBook.lua:91: in function 'IsStance'
    ...Addons\WatcherEx\GeminiSpellBook\GeminiSpellBook.lua:132: in function 'GetInnates'
    ...aming\NCSOFT\WildStar\Addons\WatcherEx\WatcherEx.lua:1868: in function 'UpdateSkillsList'
    ...aming\NCSOFT\WildStar\Addons\WatcherEx\WatcherEx.lua:1606: in function <...aming\NCSOFT\WildStar\Addons\WatcherEx\WatcherEx.lua:1600>

So I'm using these 2 functions together:

-- Description: Determines whether a given spell ID is a stance or not.
--
-- Parameters:  - spell: The Spell object from the game we need to check.
--
-- Returns:     True if the ID is that of a stance spell; otherwise false.
--
function GeminiSpellBook:IsStance(spell)
    for i,group in ipairs(ctClassStancesAbilities) do
        for j, stanceId in pairs(group) do
            if spell:GetId() == stanceId then
                return true
            end
        end
    end

    return false
end

Which is called by the following function:

-- Description: Gets all the Innate abilities for the current character being played and stores
--              them in the internal tInnates structure.
--              The function takes no arguments, but expects a 'self' parameter, so
--              should be called as SpellBook:GetInnates()
--
-- Parameters:
--
-- Returns:     The list of Innate abilities.
function GeminiSpellBook:GetInnates()
    if self.tInnates == nil or #self.tInnates <= 0 then
        self.tInnates = { }
        local tSpells = GameLib.GetClassInnateAbilitySpells().tSpells or { }

        for _,spell in ipairs(tSpells) do
            if spell ~= nil and not self.IsStance(spell) then
                table.insert(self.tInnates, spell)
            end
        end
    end

    return self.innates
end

I've added a check to see if I was passing a nill value, but still I keep getting the error. The way I interpret is that the passed parameter turns into nil during the function call.

However when I check the data in the console myself, the data is there....

Answer 1

You should change the call from self.IsStance(spell) to self:IsStance(spell) or use self.IsStance(self, spell). Since you defined IsStance as the method (using GeminiSpellBook:IsStance) it gets the first parameter as self, which is where spell gets assigned and the spell parameter you expect gets nil value, which is causing the error you see.