"Data at the root level is invalid" when performing xsl translation

public static bool SendRegistrationEmail(RegistrationBase registrantInformation)
        {
            StreamReader sr;//for debugging             
            try
            {
                XslCompiledTransform optimusPrime = new XslCompiledTransform();
                using (Stream xslStream = new FileStream(StaticValues.XslTemplatesDir + "EmailTranslation.xsl", FileMode.Open))
                {
                    //Debug the contents of the stream
                    sr = new StreamReader(xslStream);
                    string temp = sr.ReadToEnd();
                    xslStream.Position = 0;
                    
                    //Load the xsl document into compiled transformer
                    XmlReader xslTemplateReader = XmlReader.Create(xslStream);
                    optimusPrime.Load(xslTemplateReader);
                    xslTemplateReader.Close();
                    xslStream.Position = 0;  
                    
                    MailMessage confirmationEmail = new MailMessage(StaticValues.RegistrationEmailAddress, registrantInformation.Email);
                    
                    using (Stream transformedHtmlStream = new MemoryStream(), regInfoStream = new MemoryStream())
                    {
                        //create serialized object and provide a reader
                        XmlSerializer regInfoSerializer = new XmlSerializer(typeof(RegistrationBase));//Init serializer
                        XmlWriter regInfoWriter = XmlWriter.Create(regInfoStream); //Init xml writer
                        regInfoSerializer.Serialize(regInfoWriter, registrantInformation); //serialize
                        regInfoWriter.Close();
                        regInfoStream.Position = 0;                    
                        XmlReader registrationInfoReader = XmlReader.Create(regInfoStream); //provide reader for stream
                        regInfoStream.Position = 0;
 
                        //Debug contents of serialized registration information stream
                        sr = new StreamReader(regInfoStream);
                        string tempRegInfo = sr.ReadToEnd();
                        regInfoStream.Position = 0;
 
                        //Perform Transformation
                        optimusPrime.Transform(registrationInfoReader,null, transformedHtmlStream);
                        transformedHtmlStream.Position = 0; //reset stream
                        registrationInfoReader.Close();
 
                        //Read contents of transformed html into email body
                        sr = new StreamReader(transformedHtmlStream);
                        confirmationEmail.Body = sr.ReadToEnd();                    
                    }
                     //SEND THE CONFIRMATION EMAIL
                    SmtpClient smtpSender = new SmtpClient(StaticValues.SmtpHost);
                    confirmationEmail.Subject = "Email Verification";
                    confirmationEmail.IsBodyHtml = true;
                    smtpSender.Send(confirmationEmail);
                    return true;
                }
            }
            catch (Exception ex)
            {
                Helper.LogException(ex);
                return false;
            }
        }