XPSDocumentWriter WriteAsync doesn't save the FixedDocument to a xps file

I have the following function to save a report to a xps file:

public XpsDocumentWriter Save(string fileName, Action<string> onSaveCompletion, Action<string> onSaveError)
{
string xpsFileName = Path.GetTempFileName();
        xpsFileName = Path.ChangeExtension(xpsFileName, "xps");
        // make sure we delete any existing files
        File.Delete(xpsFileName);
        File.Delete(fileName);
        XpsDocument savedDocument = new XpsDocument(xpsFileName, FileAccess.ReadWrite);

        XpsDocumentWriter documentWriter = XpsDocument.CreateXpsDocumentWriter(savedDocument);

        if (documentWriter != null)
        {
                WritingCompletedEventHandler handler = null;
                handler = (sender, e) => {
                    DocumentWriter_WritingCompleted(xpsFileName, fileName, savedDocument, onSaveCompletion, onSaveError, e);
                    documentWriter.WritingCompleted -= handler;
                };

                documentWriter.WritingCompleted += handler;
                documentWriter.WriteAsync(FixedDocument);
        }
        return documentWriter;

}

When the caller is in a windows application, everything works fine. But when the caller is in a class library, nothing is write into the xps file and no WritingCompletedEvent is triggered. I also tried WritingProgressChangedEvent and it was not triggered either. In the case of class library,  synchronous call documentWriter.Write(FixedDocument) works. But I prefer to use asynchronous call since I may have a big report to save or print. Anybody have any idea how to make asynchronous call WriteAsync work with the class library? Thanks!!!