problem in exporting gridview to pdf in asp.net using c#

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ASP.NET

plz help me to fix this problem i'm facing with exporting gridview to pdf..

(itextsharp.dll have been used in coding)

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

code:

StringReader sr = new StringReader(sw.ToString());
       Document pdfDoc = new Document(PageSize.A4, 20f, 20f, 20f, 20f);
       HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
       PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
       pdfDoc.Open();
      ----> htmlparser.Parse(sr); <------- ERROR
       pdfDoc.Close();
       Response.Write(pdfDoc);
       Response.End();