OK, what am I missing here?

I have two files: BALANCE and PTYPES

PTYPES is ordered on the basis of its TYPE field

BALANCE is related to PTYPES through the TYPE field

Every TYPE in BALANCE should have a match in PTYPES.

I deliberately alter a TYPE in BALANCE to ensure that at least one record does not have a match.

If I SET FILTER TO EMPTY(PTYPES.TYPE) I duly find the erroneous record.

If I SELECT BALANCE.* FROM BALANCE,PTYPES WHERE BALANCE.TYPE=PTYPES.TYPE AND NOT EMPTY(PTYPES.TYPE) INTO DBF ANYTABLE

it duly selects all the correct records and omits the faulty one.

Yet, if I try to do the opposite (in order to detect the faulty one)
i.e.

SELECT BALANCE.* FROM BALANCE,PTYPES WHERE BALANCE.TYPE=PTYPES.TYPE AND EMPTY(PTYPES.TYPE) INTO DBF ANYTABLE

I get zilch.

Pourquoi?

Harry