sorry - too fast on the keyboard...

I have an update statement which is trying to do the following

UPDATE TBLBSLIST SET
BSREQIND = :BSREQIND, /*PK*/
LINKTOPROJ = (SELECT LINKTOPROJ FROM TBLJOBSCHEDULE WHERE
TBLJOBSCHEDULE.JOBIND=:LINKTOJOBSCHED),
LINKTOWO = (SELECT LINKTOBSAWWO FROM TBLJOBSCHEDULE WHERE
TBLJOBSCHEDULE.JOBIND=:LINKTOJOBSCHED),
BSNO = :BSNO,
LINKTOSLAB = :LINKTOSLAB,
etc

the result is that the first LINTOPROJ value is set correctly, but it apears
as though the second use of the parameter :LINKTOJOBSCHED is NULL.
Is this supposed to happen, i.e. a parameter is cleared to null after it's
first use?
Is this IBO or the engine?

Alan