trouble with sum

select sum(P.price), P.product_id, S.sid, S.product_id from products P,
shoppers S
where S.sid='0319199910043810.3.0.12' and P.product_id= S.product_id;

i've used similar commands like this one with other engines with no
trouble

what do i need to do to get it to work with postgre


thanks

pete