:42: Attempt to perform arithmetic on local 'y' (a nil value)

local rect = graydraw.rect
local setColor = graydraw.setcolor
local wait = misc.wait
local line = graydraw.line

screenWidth = 128
screenHeight = 64

key_left = 37
key_right = 40
key_exit = 36
gDoorPosition = {0,0,0}
gDoorActive = 1

function main()
    setColor(1)
    menu()
end

function menu()
    while not key(key_exit) do
        if key(key_left) then gDoorActive = gDoorActive - 1 end
        if key(key_right) then gDoorActive = gDoorActive + 1 end
        if gDoorActive < 1 then gDoorActive = 1 end
        if gDoorActive > 3 then gDoorActive = 3 end

        for i = 1, 3, 1 do
            if i == gDoorActive and gDoorPosition[i] < 30 then gDoorPosition[i] = gDoorPosition[i] + 1
            elseif gDoorPosition[i] > 0 then gDoorPosition[i] = gDoorPosition[i] - 1 end
        end

        for ix = 1, 3, 1 do
            garageDoor(ix * 20 + 5, y, 20, 30, gDoorPosition[ix], 5)
        end

        refresh
    end
end

function  garageDoor(x, y, width, height, position, panelHeight)

    rect(x, y, x + width, y + height, 4, 4) --draw garage outine
    rect(x, y, x + width, y + height - position, 4, 0) --draw door

    --draw panels
    for iy = height - position, 0, -panelHeight do
        line(x, y + iy, x + width, y + iy, 4)
    end
end

main()