trouble with unique constraint

The following SQL leads to a unique constraint violation error message (PostgreSQL 8.4.1).

create table test (val integer);

create unique index test_uni on test(val);

insert into test (val) values (1);

insert into test (val) values (2);

insert into test (val) values (3);

update test set val = val + 1;

But it works fine with Oracle, MSSQL and DB2. Any idea how to make it working with PostgreSQL as well?