把数据库中的数据以excel文件的格式显示在浏览器中的错误

前台内容:
<body>
    <form id="form1" runat="server">
    <div>
        <asp:GridView ID="gv" runat="server">
        </asp:GridView>
    
    </div>
    </form>
</body>
后台内容:        conn.Open();
        SqlDataAdapter da = new SqlDataAdapter();
        DataSet ds = new DataSet();
        da.SelectCommand =new SqlCommand("select * from kc_table",conn);
        da.Fill(ds);
        DataView dv = new DataView(ds.Tables[0]);
        gv.DataSource = dv;
        gv.DataBind();
        conn.Close();
        conn.Dispose();
        conn = null;
        if (Request.QueryString["bExcel"] == "1")
        {
            Response.ContentType = "application/vnd.ms-excel";
            Response.Charset = "";
            EnableViewState = false;
            System.IO.StringWriter tw = new System.IO.StringWriter();
            System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
            gv.RenderControl(hw);
            Response.Write(tw.ToString());
            Response.End();
        }
错误信息:
用户代码未处理HttpException
类型"gridview"控件"gv"必须放在具有runat=server的窗体标记内
谢谢各位朋友的点拨