Word fails to save custom document properties

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' Launch Word
   Dim wdapp As Object
   Set wdapp = CreateObject("Word.Application")

   ' Make it visible...
   wdapp.Visible = True

   ' Open a document...
   Dim doc As Object<?xm-insertion_mark_start author="v-thomr" time="20070416T123039-0600"?>
<?xm-insertion_mark_end?>   ' In Word 2007, change the document name to testdoc.docx.
   Set doc = wdapp.Documents.Open("C:\mydocu~1\testdoc.doc")

   ' Add a custom document property...
   doc.CustomDocumentProperties.Add "MyProp", False, 1, 123

   ' Uncomment this line to let Word know document is "dirty," so it
   ' will save the new Custom Document Property.
   'doc.Saved = False

   ' Save document
   doc.Save

   ' Close Word...
   wdapp.Quit