Exception during Export to Excel?

I’m trying to export data from DataGridView into Excel sheet, so I used the following code, but at bolded and underlined line I’m getting an exception that says “{"RegisterForEventValidation can only be called during Render();"}”

How should I solve this?

private void ExportGridView()

    {

        try

        {

            string attachment = "attachment; filename = Report.xls";

            Response.ClearContent();

            Response.AddHeader("content-disposition", attachment);

            Response.ContentType = "application/ms-excel";

            StringWriter sw = new StringWriter();

            HtmlTextWriter htw = new HtmlTextWriter(sw);

            HtmlForm frm = new HtmlForm();

            GridView1.Parent.Controls.Add(frm);

            frm.Attributes["runat"] = "server";

            frm.Controls.Add(GridView1);

            frm.RenderControl(htw);

            //GridView1.RenderControl(htw);

            Response.Write(sw.ToString());

            Response.End();

        }

        catch (Exception e1)

        {

            Response.Write(e1);

        }

    }

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