为什么传递过来的参数为NULL

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            VoteClassDataContext db = new VoteClassDataContext();
            var votes = (from v in db.VoteItem where v.TitleID == Int32.Parse(
                             Request.QueryString["titleid"].ToString()) select v.ItemCount).Sum();
            VoteCount = votes;
        }
}

错误提示：用户代码未处理 System.NullReferenceException
  Message="未将对象引用设置到对象的实例。"