lua:尝试对字段'?'（空值）进行索引

现在我在下面的代码中一直遇到这个错误：

box.lua:31: 尝试对字段'?'（空值）进行索引

我试图纠正这个错误，但失败了。

以下是完整的代码：

    function ChatSystem(event, player, msg, Type, lang)
        local Guid = player:GetGUIDLow()
        if flag[Guid] ~= 0 then
            local daan = quiz["box"][flag[Guid]][2];
            local t = msg;
            if (t+1 == daan+1) then
                player:SendBroadcastMessage("[注意]正确")
                SetPlayerFlag(player, 0)
                SetPlayererror(player, 0)
            else
                cuowu[Guid] = error[Guid] + 1;
                if cuowu[Guid] >= quiz["frequency"] then
                    SetPlayererror(player, 0)
                    Kick( player )
                else
                    player:SendBroadcastMessage("[注意]  错误"..cuowu[Guid].."   "..quiz["box"][flag[Guid]][1].."");
                end
            end
        end
    end


    function Start (event, player, item, count)
        local datigailv = math.random(1, 100)
        if (datigailv <= quiz["Chance"])then
            local select = math.random(1, 5)
            local Guid = player:GetGUIDLow()
            SetPlayerFlag(player, select)
            SetPlayererror(player, 0)
            player:SendBroadcastMessage("[注意]回答我的问题："..quiz["box"][flag[Guid]][1].."");
        end
    end

    RegisterPlayerEvent(32, Start)
    RegisterPlayerEvent(18, ChatSystem);