CASCADE PB

Hello,
I use postgreSQL 7.2.3 and I have trouble on deleting.
I would like to automaticly delete line in the 'coop' table when I dlete the
line in the 'fact' table but it is not ok.
Where is my mistake ?
This is my database:

create table coop (
id_coop serial not null,
nom_coop varchar(50) not null,
primary key(id_coop)
);

create table fact (
id_fact serial not null,
id_coop integer references coop(id_coop) on delete cascade,
date_fact timestamp not null,
montant_fact decimal(6,3),
primary key(id_fact)
);

Thank's for your support.