"Invalid index/tag name."

I have been struggling with this all day.  I am trying to create a secondary
index on a Paradox database table, and I get the message "Invalid index/tag
name.  Index: SENSORS"  Here is the latest version of the code:

  TACopy.Mode := batCopy;
     TACopy.Execute;            {copy the dataset from SensorsLibraryQuery}
     TATable.Close;
     TATable.EmptyTable;        {now we will add an index to remove dupes}
     TATable.AddIndex('WTA', 'Contact_ID;Sensor', [ixPrimary]);
     TACopy.KeyViolTableName := 'DupTAs';
     TACopy.AbortOnKeyViol := False;
     TACopy.Mode := batAppend;
     TACopy.Execute;            {append the same dataset which removes dupes}
     TATable.AddIndex('SENSORS', 'Sensor', []);  
     TATable.IndexName := 'Sensors';  {now we apply the index we really want}
     TATable.Active := True;

It consistantly blows up on the second AddIndex, even though I swear I had this
code working yesterday.  I started by creating the dataset and using IndexDefs,
but that blew up the same way.