此页的状态信息无效,可能已损坏。 这个是什么错 ？

<asp:DropDownList ID="DropDownList1" runat="server" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
                                AutoPostBack="true">
                            </asp:DropDownList>
                            <asp:DropDownList ID="DropDownList2" runat="server" OnSelectedIndexChanged="DropDownList2_SelectedIndexChanged"
                                AutoPostBack="true">
                            </asp:DropDownList>
                            <asp:DropDownList ID="DropDownList3" runat="server">
                            </asp:DropDownList>
 
 protected void Page_Load(object sender, EventArgs e)
        {
            if (!this.IsPostBack)
            {
                Bind();
            }private void Bind()
        {
            DataTable dt = TruckRules.TruckChinaProvinceRules.SelectChinaProvinceList();            if (dt != null)
            {
                foreach (DataRow dr in dt.Rows)
                {
                    this.DropDownList1.Items.Add(new ListItem(dr["F_ProvinceName"].ToString(), dr["F_ProvinceId"].ToString()));
                }
            }
        }
        protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
        {
            string F_ProvinceId = this.DropDownList1.SelectedValue;            DataTable dt = TruckRules.TruckChinaProvinceRules.SelectProvinceByCityPidFill(F_ProvinceId);
            // ddl1.Items.Clear();
            if (dt != null)
            {
                foreach (DataRow dr in dt.Rows)
                {
                    this.DropDownList2.Items.Add(new ListItem(dr["F_CityName"].ToString(), dr["F_CityId"].ToString()));
                }
            }
        }
        protected void DropDownList2_SelectedIndexChanged(object sender, EventArgs e)
        {
            string F_CityId = this.DropDownList2.SelectedValue;            DataTable dt = TruckRules.TruckChinaProvinceRules.GetCityByAreasFill(F_CityId);
            //ddl2.Items.Clear();
            if (dt != null)
            {
                foreach (DataRow dr in dt.Rows)
                {
                    this.DropDownList3.Items.Add(new ListItem(dr["F_AreaName"].ToString(), dr["F_Id"].ToString()));
                }
            }
        }
}
我就纳闷了  这个空间的下拉框  单独放在一个页面就能使  一放到我的那个页面就有错 报错内容： 此页的状态信息无效,可能已损坏。大家帮忙看一下  纠结呢   谢谢