strange sum behaviour

Hi,

I am getting strange results from the sum function as follows:

corp=# select amount from acc_trans where trans_id=19721 and chart_id=10019;
 amount
---------
    4.88
   117.1
 -121.98
(3 rows)

corp=# select sum(amount) from acc_trans where trans_id=19721 and
chart_id=10019;
         sum
----------------------
 -1.4210854715202e-14
(1 row)


amount is defined as double precision. I noticed that if I cast amount
as numeric, the sum comes out 0 as expected.

I am using postgresql 8.0.1

Thanks in advance for any help or suggestions.

Andrew Baerg