Creating unique constraints on OID

Shouldn't this work?

create table test ( a int, unique (oid) );
ERROR: CREATE TABLE: column "oid" named in key does not exist

Because this works:

create table test ( a int );
CREATE

alter table test add unique (oid);
NOTICE: ALTER TABLE/UNIQUE will create implicit index 'test_oid_key' for table 'test'
CREATE

And shouldn't the last one say "ALTER"?

--
Peter Eisentraut peter_e(at)gmx(dot)net http://funkturm.homeip.net/~peter