UNIQUE KEY with null value...

Hi,
I´m a brazilian programmer!

I have a question about Postgres UNIQUE KEY. Look:

(Step 1)
CREATE TABLE test (
id integer not null primary key,
id_uni integer not null,
cod integer,
name varchar(40),
constraint test_uk unique(id_uni, cod)
)

(Step 2)
insert into test (id, id_uni, cod, name)
values (1, 1, null, 'A')

(Step 3)
insert into test (id, id_uni, cod, name)
values (2, 1, null, 'B') =========> [Have the same UNIQUE KEY]

Why postgresql don´t break insertion in this case????

Thanks
Diego Ziquinatti
Brazil