got non-convertible string

[SoManyTears]

hi guys 

Can you tell me why I got "warning" from these little codes?

warning : Expected numbers,got non-convertable string.This warning may be an error in future versions.

 

function showSpecificSlotWindow( slotid )
    local slotname = getVehicleUpgradeSlotName( slotid )
    if type( slotid ) == 'string' then
        slotname = slotid
    end
    local upgrades = getVehicleCompatibleUpgrades( moddingVeh )
    if not hideSubToo then
        guiSetText( upgradeGUI.wnd, slotname )
        showNewSub = false 
    else
        addEventHandler( "onClientRender", getRootElement(), contractSubWindow )
    end
    layoutButtons( slotname )
end