从FlowDocument创建一个XPS文档，并动态地附加它

我有一个FlowDocument，我想转换为XPS文档，并将其附加到电子邮件中，并将其一起发送。我使用这个代码

 public static MemoryStream FlowDocumentToXPS(FlowDocument flowDocument, int width, int height)
    {
        MemoryStream stream = new MemoryStream();
        using (Package package = Package.Open(stream, FileMode.Create, FileAccess.ReadWrite))
        {
            using (XpsDocument xpsDoc = new XpsDocument(package, CompressionOption.Maximum))
            {                  
                XpsSerializationManager rsm = new XpsSerializationManager(new XpsPackagingPolicy(xpsDoc), false);
                DocumentPaginator paginator = ((IDocumentPaginatorSource)flowDocument).DocumentPaginator;
                paginator.PageSize = new System.Windows.Size(width, height);
                rsm.SaveAsXaml(paginator);
                rsm.Commit();                
            }
        }
        stream.Position = 0;
        Console.WriteLine(stream.Length);
        Console.WriteLine(stream.Position);
        return stream;   
    }

然后我用下面的代码附加它:

Attachment xps = new Attachment(FlowDocumentToXPS(FD, 768, 676), "FileName.xps", "application/vnd.ms-xpsdocument");

其中FD是我想转换的FlowDocument，我收到了0.0KB大小的XPS文件附件，它不能用XPS查看器打开，我在这里缺少什么?

编辑:最后的代码工作，见注释

Thanks in advance