I am new using postgresql and have problem when sum record, because give a zero result.

this my db table

user_id   amount
981       1200000.0000
981       -170000.0000
981       -144500.0000
981       -249800.0000

the amount field using "numeric" datatype and this my query

SELECT user_id, SUM(amount) as amount FROM transaction where user_id = 981;

how to fix this, thanks