PDF盖章在没有标题信息或正文的页面上不起作用

使用以下代码pdf盖章可以正常工作，但如果pdf页面没有标题信息或正文为空，则盖章不起作用。我的意思是pdfData.ShowText（strCustomMessage）不起作用。

//create pdfreader object to read sorce pdf
PdfReader pdfReader=new PdfReader(Server.MapPath("Input") + "/" + "input.pdf");
//create stream of filestream or memorystream etc. to create output file
FileStream stream = new FileStream(Server.MapPath("Output") + "/output.pdf",  FileMode.OpenOrCreate);
//create pdfstamper object which is used to add addtional content to source pdf file
PdfStamper pdfStamper = new PdfStamper(pdfReader,stream);
//iterate through all pages in source pdf
for (int pageIndex = 1; pageIndex <= pdfReader.NumberOfPages; pageIndex++)
{
//Rectangle class in iText represent geomatric representation... in this case, rectanle object would contain page geomatry
Rectangle pageRectangle = pdfReader.GetPageSizeWithRotation(pageIndex);
//pdfcontentbyte object contains graphics and text content of page returned by pdfstamper
PdfContentByte pdfData = pdfStamper.GetUnderContent(pageIndex);
//create fontsize for watermark
pdfData.SetFontAndSize(BaseFont.CreateFont(BaseFont.HELVETICA_BOLD, BaseFont.CP1252, BaseFont.NOT_EMBEDDED), 40);
//create new graphics state and assign opacity
PdfGState graphicsState = new PdfGState();
graphicsState.FillOpacity = 0.4F;
//set graphics state to pdfcontentbyte
pdfData.SetGState(graphicsState);
//set color of watermark
pdfData.SetColorFill(BaseColor.BLUE);
//indicates start of writing of text
pdfData.BeginText();
//show text as per position and rotation
pdfData.SetTextMatrix(pageRectangle.Width/2,pageRectangle.Height/2);                                                                                     pdfData.SetFontAndSize(BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED), 9);
string strCustomMessage="PDF Stamping Test"
pdfData.ShowText(strCustomMessage);
//call endText to invalid font set
pdfData.EndText();
}
//close stamper and output filestream
pdfStamper.Close();
stream.Close();