can't delete documents

[Michael Paine]

Hi, I have been having problems trying to delete documents within an index.
Whenever I execute the following:
 
        Dim reader As IndexReader = IndexReader.Open(DefaultStore)
        Dim deleted As Integer = reader.DeleteDocuments(New
Term("MessageId", CStr(MessageId)))
        reader.Close()

whereby MessageId is always unique, deleted always equals 0.  Do I need to
do something in particular before this will work?
 
Thank you,
Michael