Why do I get an error message in VFP 8.0 in regard to the SUM function? The error message says that sum.prg does not exist.

SELECT ;
APTRXFIL.VOUCHER_NO, ;
APTRXFIL.INV_AMT, ;
SUM(APTRXDST.DST_AMT) AS DST_AMT ;
FROM ;
APTRXFIL, ;
APTRXDST ;
WHERE ;
APTRXFIL.VOUCHER_NO = APTRXDST.VOUCHER_NO AND;
APTRXFIL.INV_AMT <> SUM(APTRXDST.DST_AMT) ;
GROUP BY ;
APTRXFIL.VOUCHER_NO, ;
APTRXFIL.INV_AMT ;
INTO CURSOR tmpAPTransactions