Unknown Error, how to fix?

So, expanding on my last question, I solved it, I accidentally added a false value, but I recently got a NEW error...

Workspace.TrelloAPI:547: attempt to concatenate local 'des' (a userdata value)

17:53:48.905 - Script 'Workspace.TrelloAPI', Line 547 - method AddCard

17:53:48.906 - Script 'Players.Player.PlayerGui.Application.Frame.create.Script', Line 16

Line 547:

desc=des.."",

Line 16:

 local CardID = ap:AddCard("[NS] " .. group.Name, id, ListID)

AddCard

function T:AddCard(...)
        local url
        local args={...}
        local dat
        if #args==3 then
            local nam,des,lid=args[1],args[2],args[3]
            dat={
            name=nam,
            desc=des.."",
            idList=lid,
            urlSource=nil,
            due=nil
            }
        elseif #args==4 then
            local nam,des,lid,url=args[1],args[2],args[3],args[4]
            dat={
            name=nam,
            desc=""..des,
            idList=lid,
            urlSource=url,
            due=nil
            }
        elseif #args==5 then
            local nam,des,lid,url,po=args[1],args[2],args[3],args[4],args[5]
            dat={
            name=nam,
            desc=""..des,
            idList=lid,
            urlSource=url,
            pos=po,
            due=nil
            }
        elseif #args==6 then
            local nam,des,lid,url,po,de=args[1],args[2],args[3],args[4],args[5],args[6]
            dat={
            name=nam,
            desc=""..des,
            idList=lid,
            urlSource=url,
            pos=po,
            due=de
            }
        elseif #args==7 then
            local nam,des,lid,url,po,de,label=args[1],args[2],args[3],args[4],args[5],args[6],args[7]
            dat={
            name=nam,
            desc=""..des,
            idList=lid,
            urlSource=url,
            pos=po,
            due=de,
            labels=label
            }
        elseif #args==8 then
            local nam,des,lid,url,po,de,label,cc=args[1],args[2],args[3],args[4],args[5],args[6],args[7],args[8]
            dat={
            name=nam,
            desc=""..des,
            idList=lid,
            urlSource=url,
            pos=po,
            due=de,
            labels=label,
            idCardSource=cc
            }
        end
    local data=HS:JSONEncode(dat)
    if Private then
            getAddon()
            url="https://api.trello.com/1/cards"..addon
        else
             getAddon()
             url="https://api.trello.com/1/cards"..addon
        end
        local re=HS:PostAsync(url,data)
        return HS:JSONDecode(re)
    end