String variable constraints

I have a Base64 jpeg image string that is a simple signature image. I can store a string in SQL Server and return it, but when I try to pass it to a method or store it in a Session variable, I get null. Is there a limit to the string you can pass to a method or store in a var session?

Here is the code;

I am getting string from db,

      string VSignature = ds.Tables[0].Rows[0]["SignatureB64"];

// VSignature gets valued ok
// then passed to a class with method to handle image,

      Write.GetPageOneReadyToBeAltered(f_older + "\\FDD.PDF", f_older + @"\N.PDF", 
            CertID, VSignature);

      

        
        
        
      

    

// VSignature is null here

      public static void GetPageOneReadyToBeAltered(string PageNReader, string PageNStamper, string CertificateNo, string VSignature)
        {
        // prepare page one copy to be altered by user  

        PdfReader pdfreader = new PdfReader(PageNReader);
        PdfStamper pdfStamper = new PdfStamper(pdfreader, new FileStream(PageNStamper, FileMode.Create));
            /*
             some pdf stuff done here, irrelevant
            */
            var pdfContentByte = pdfStamper.GetOverContent(1);

             byte[] bytSig1 = Convert.FromBase64String(VSignature);

            MemoryStream msSig1 = new MemoryStream(bytSig1);
            iTextSharp.text.Image sig1 = iTextSharp.text.Image.GetInstance(msSig1);
            sig1.SetAbsolutePosition(23, 76);
            sig1.ScaleToFit(60f, 60f);
            pdfContentByte.AddImage(sig1);
       }

      

        
        
        
      

    

thank