Interesting failure when selecting aggregates

select sum(TotShippingHandling) from Invoice where 1 = 1 and
TotShippingHandling <> 0;
---

(1 row)

select sum(TotShippingHandling) from Invoice where TotShippingHandling <> 0;
sum
-------
6781.05
(1 row)

Red Hat 5.1, intel, PostgreSQL 6.5 (downloaded last night).

Thanks, Michael