Cast question (NULL -> NUMERIC)

I have the following query:

    SELECT cast( sum(amount) AS NUMERIC(9,2) ) ...
    WHERE postdate < $startdate

(where $startdate is replaced with the user's entry at runtime) which returns
the balance forward of the amount column for the user-suplied date correcly
unless the user supplies a $startdate such that no culumns are being added.
In this case NULL is returned. I want to receive 0.00 in this case instead of
NULL, so I added the cast above, but NULL is still returned. Any ideas?

--
Henry House
OpenPGP key available from http://romana.hajhouse.org/hajhouse.asc