error: the document has no page when to upload pdf file

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C#

ASP.NET

using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.text.html;
using iTextSharp.text.html.simpleparser;

Response.ContentType = "application/pdf";

        Response.AddHeader("content-disposition", "attachment;filename=Report1.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        HtmlForm frm = new HtmlForm();
        gd_dt.Parent.Controls.Add(frm);
        frm.Attributes["runat"] = "server";
        frm.Controls.Add(gd_dt);
        frm.RenderControl(hw);
        StringReader sr = new StringReader(sw.ToString());
        Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);
        HTMLWorker htmlparser = new HTMLWorker(pdfDoc);

        PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
        pdfDoc.Open();
        htmlparser.Parse(sr);
       
        pdfDoc.Close();
        Response.Write(pdfDoc);
        Response.End();

this is the code i'm using in button upload.im getting this error.i dont know why it is happen.pls guys help me..



thank in advance
musiw.