create unique index doesn't work properly

when executing the code bellow:

---
create table test_unique(i1 integer, i2 integer, unique(i1,i2));
insert into test_unique(1,null);
insert into test_unique(1,null);
insert into test_unique(1,null);
---

all "inserts" terminate sucefully, if there isn't "null" values it works
fine.