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();