无法强制转换类型为'；的对象；iTextSharp.text.html.simpleparser.CellWrap

在将GridView数据转换为PDF时，我收到此错误。请帮帮我。我的密码在下面。

Response.Cache.SetCacheability(HttpCacheability.NoCache);
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
HtmlForm frm = new HtmlForm();
gv.Parent.Controls.Add(frm);
frm.Attributes["runat"] = "server";
frm.Controls.Add(gv);
frm.RenderControl(hw);
StringReader sr = new StringReader(sw.ToString());
Document pdfDoc = new Document(PageSize.A4, 30f, 30f, 30f, 0f);
HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
pdfDoc.Open();
htmlparser.Parse(sr);
pdfDoc.Close();
Response.Write(pdfDoc);
Response.End();