export gridview to excel ......error

Question

 protected void Button1_Click(object sender, EventArgs e)

    {

         Response.Clear();

        Response.AddHeader("content-disposition", "attachment;filename=GridView_to_Excel.xls");

        Response.Charset = "";

        Response.ContentType = "application/vnd.xls";

        StringWriter StringWriter = new System.IO.StringWriter();

        HtmlTextWriter HtmlTextWriter = new HtmlTextWriter(StringWriter);

        

        gv.RenderControl(HtmlTextWriter);

        Response.Write(StringWriter.ToString());

        Response.End();

        }

    }

.....................................................................................................

error .....

Exception Details: System.Web.HttpException: Control 'gv' of type 'GridView' must be placed inside a form tag with runat=server.

Source Error: 

Line 122:        HtmlTextWriter HtmlTextWriter = new HtmlTextWriter(StringWriter);
Line 123:        //GridView1.RenderControl(HtmlTextWriter);
Line 124:        gv.RenderControl(HtmlTextWriter);
Line 125:        Response.Write(StringWriter.ToString());
Line 126:        Response.End();