Can't delete documents

[Gautam Lad]

Hi there
 
I have indexed my documents using various keys, and one of the keys is an ISBN.
 
ISBNs can be all numeric or have an X at the end, (eg. 123456789X or 1234567890)
 
Each of my documents is added to an index like so:
  doc.Add(New Field(key.Name, key.Value, Field.Store.YES, 
Field.Index.TOKENIZED))
  Here, key.Name = "isbn" and key.value = "123456789X"
   
When I delete documents I use the following piece of code:
 
    Public Sub DeleteSpecificKey()
        Dim LReader As Index.IndexReader = 
Index.IndexReader.Open(Store.FSDirectory.GetDirectory(IndexDir, False))
        Dim iResult As Integer
          iResult = LReader.DeleteDocuments("isbn", "123456789X"))
          LReader.Close()
    End Sub
  
However, since there’s an “X” in the key value, it doesn’t delete it.   If the 
value was all numeric it works fine.
  What am I missing here? Everything else works fine as far as searching goes.
  Thanks,