Locate doesn't work correctly

I have a table with 3 fields. My field1 and field2 are table's
primary key. I have a secondary index on field3. See the
following code:
procedure TForm1.Button1Click(Sender: TObject);
begin
Table1.IndexFieldNames := 'FIELD3';
Table1.Locate('FIELD1; FIELD2',VarArrayOf([1998,10]),[]);
end;
During execution of method locate is raised a exception
EDBEngineError "Invalid field descriptor".
If the current index of Table is the primary key index the locate
work's without problem.
I am using:
- Oracle 8.0.4,
- Delphi 4.0 (Build 5.37)
- BDE 5.00
Thank's
Fbio