c# - XMLWorker Exception : オブジェクト参照がオブジェクトのインスタンスに設定されていません

Question

HTMLをPDFにエクスポートしたい

 var document = new Document();
 PdfWriter pdfWriter = PdfWriter.GetInstance(document, 
                                   new FileStream("/my.pdf", FileMode.Create)); 
  pdfWriter.SetFullCompression();
  pdfWriter.StrictImageSequence = true;
  pdfWriter.SetLinearPageMode();

 var sr = new StringReader(htmlcode);
document.Open();

var k = XMLWorkerHelper.GetInstance();
k.ParseXHtml(pdfWriter, document, sr);
//here it gave me an exception: Object reference not set to an instance of 
                                                                  an object


  sr.Close();
   document.Close();

Response.ContentType = "Application/pdf";
Response.AppendHeader("Content-Disposition", "attachment; filename="my.pdf");
Response.TransmitFile(@"c:\test\my.pdf");
Response.Flush();
Response.End();
Response.Close();

すべての例外:

 System.NullReferenceException: Object reference not set to an 
instance of an object.
       at iTextSharp.tool.xml.pipeline.html.HtmlPipeline.Close
                          (IWorkerContext context, Tag t, ProcessObject po)
       at iTextSharp.tool.xml.XMLWorker.EndElement(String tag, String ns)
       at iTextSharp.tool.xml.parser.XMLParser.EndElement()
    at iTextSharp.tool.xml.parser.state.ClosingTagState.Process(Char character)
       at iTextSharp.tool.xml.parser.XMLParser.ParseWithReader(TextReader reader)
       at iTextSharp.tool.xml.XMLWorkerHelper.ParseXHtml(PdfWriter writer,
                                                    Document doc, TextReader inp)