YEAR is not unique and must be qualified.

I get the above error msg when running the following code...

SELECT ;
  A.Invoice, ;
  ShipDate, ;
  Oslsm, ;
  Prodct, ;
  Cust, ;
  QtyOrd, ;
  Oprice, ;
  QtyUpd, ;
  Ocompc, ;
  A.ShipDue, ;
  Terr, ;
  WrHse, ;
  Scity, ;
  Sstate, ;
  A.Year, ;
  A.Month ;
FROM ;
  OrdrDtlH A, OrdrHdrH ;
INTO TABLE ShipHst ;
WHERE ;
  A.Invoice = OrdrHdrH.Invoice .AND. ;
  Year = '1999' .AND. ;
  Month = '12'

Yes, even though it seems odd, Year and Month are Character fields.
Haven't been able to resolve problem. Would appreciate any help. TIA, Chuck