Cannot create PdfStamper (NullReferenceException)

Question

I have a specific problem that I cannot seem to resolve.

I am trying to create a PdfStamper with the following code.

The line marked with --> is where the exception pops up.

PdfReader pdfReader = new PdfReader(SourcePdfFileName);
byte[] arr;

using (MemoryStream ms = new MemoryStream())
{
  PdfStamper pdfStamper;
  --> pdfStamper = PdfStamper.CreateSignature(pdfReader, ms, '\0');
  arr = ms.ToArray();
}

and the other method I tried

PdfReader pdfReader = new PdfReader(SourcePdfFileName);
FileStream signedPdf = new FileStream(DestPdfFileName, FileMode.Create, FileAccess.Write);

--> PdfStamper pdfStamper = new PdfStamper.CreateSignature(pdfReader, signedPdf, '\0');

Both methods throw a NullReferenceException.

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=itextsharp

StackTrace:
at iTextSharp.text.Version.GetInstance()

The original file exists on the path "SourcePdfFileName" and a blank pdf file is created when the file stream is opened. What am I missing ?

Edit: referring to this answer.

None of the parameters used in PdfStamper.CreateSignature() are null. The only null field here is the variable pdfStamper. I have tried initializing it with

PdfReader pdfReader = new PdfReader(SourcePdfFileName);
FileStream signedPdf = new FileStream(DestPdfFileName, FileMode.Create, FileAccess.Write);
PdfStamper pdfStamper = new PdfStamper(pdfReader,signedPdf);

which in return throws the same exception mentioned above.