attempt to call method 'insert' (a nil value)

Question

How come I get a attempt to call method 'insert' (a nil value) error on the line containing insert ?

Changing it to instance.sprites = bg does make it work, but I want to return all sprites in a separate table (sprites).

local writingTool = {}

local _W, _H = display.contentWidth, display.contentHeight

function writingTool:new()
    local instance = {}
    instance.index = writingTool
    setmetatable(instance, self)

    instance.sprites = {}

    local bg = display.newImage("images/backgrounds/wooden_bg.png")
    bg.x = _W/2
    bg.y = _H/2
    instance.sprites:insert(bg)
    return instance
end

return writingTool

Edit: Trying instance.sprites.bg = bg does not work either. Give this error:

bad argument #-2 to 'insert' (Proxy expected, got nil)

Answer 1

instance.index = writingTool

Should be

instance.__index = writingTool

Though I would remove the above line and implement it in the one below like so:

setmetatable(instance,{__index=writingTool})

Also, t:insert() or t.insert() aren't defined by default , to insert elements into a table you use the table.insert function as defined below:

table.insert (table, [pos,] value)

so you should have table.insert(instance.sprites,bg) . So with these modifications your function should look like:

function writingTool:new()
    local instance = { sprites = {} }
    setmetatable(instance, {__index = wirtingTool})
    local bg = display.newImage("images/backgrounds/wooden_bg.png")
    bg.x = _W/2
    bg.y = _H/2
    table.insert(instance.sprites,bg)
    return instance
end