exporting gridview data to PDF error

Unable to cast object of type 'iTextSharp.text.html.simpleparser.CellWrapper' to type 'iTextSharp.text.Paragraph'.

 Response.ContentType = "application/pdf";
        Response.AddHeader("content-disposition", "attachment;filename=UserDetails.pdf");
        Response.Cache.SetCacheability(HttpCacheability.NoCache);
        StringWriter sw = new StringWriter();
        HtmlTextWriter hw = new HtmlTextWriter(sw);
        UserListGrid.AllowPaging = false;
        FillGrid();
        UserListGrid.DataBind();
        UserListGrid.RenderControl(hw);
        UserListGrid.HeaderRow.Style.Add("width", "15%");
        UserListGrid.HeaderRow.Style.Add("font-size", "10px");
        UserListGrid.Style.Add("text-decoration", "none");
        UserListGrid.Style.Add("font-family", "Arial, Helvetica, sans-serif;");
        UserListGrid.Style.Add("font-size", "8px");
        StringReader sr = new StringReader(sw.ToString());
        Document pdfDoc = new Document(PageSize.A2, 7f, 7f, 7f, 0f);
        HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
        PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
        pdfDoc.Open();
        htmlparser.Parse(sr);------> Error
        pdfDoc.Close();
        Response.Write(pdfDoc);
        Response.End();