ERROR:Control 'ContentPlaceHolder1_gridreport' of type 'GridView' must be placed inside a form tag with runat=server.

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ASP.NET

ERROR:Control 'ContentPlaceHolder1_gridreport' of type 'GridView' must be placed inside a form tag with runat=server.

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protected void btnexport_Click(object sender, EventArgs e)
  {

      string attachment = "attachment; filename=Emp.xls";
      Response.ClearContent();
      Response.AddHeader("content-disposition", attachment);
      Response.ContentType = "application/ms-excel";
      StringWriter sw = new StringWriter();
      HtmlTextWriter htw = new HtmlTextWriter(sw);
      gridreport.RenderControl(htw);
      Response.Write(sw.ToString());
      Response.End();

  }

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">

<asp:Panel ID="Panel1" runat="server" Height="317px" ScrollBars="Both">

        <asp:GridView ID="gridreport" runat="server" BackColor="White"
            BorderColor="#CCCCCC" BorderStyle="None" BorderWidth="1px" CellPadding="3"
            Height="294px" Width="418px">
            <FooterStyle BackColor="White" ForeColor="#000066" />
            <HeaderStyle BackColor="#006699" Font-Bold="True" ForeColor="White" />
            <PagerStyle BackColor="White" ForeColor="#000066" HorizontalAlign="Left" />
            <RowStyle ForeColor="#000066" />
            <SelectedRowStyle BackColor="#669999" Font-Bold="True" ForeColor="White" />
            <SortedAscendingCellStyle BackColor="#F1F1F1" />
            <SortedAscendingHeaderStyle BackColor="#007DBB" />
            <SortedDescendingCellStyle BackColor="#CAC9C9" />
            <SortedDescendingHeaderStyle BackColor="#00547E" />
        </asp:GridView>

    </asp:Panel>

</asp:Content>