DOMAIN constraints bug or feature

Hi.

Have next problem DOMAIN constraints ommited in table definition.

--- TEST FILE ---
create domain ID_DOMAIN integer not null;
create table test (
ID ID_DOMAIN,
NOTES text NOT NULL
);

\d test;

alter table test add constraint test_pk primary key (ID);

--- OUTPUT ---
CREATE DOMAIN
CREATE TABLE
Table "public.test"
Column | Type | Modifiers
--------+-----------+-----------
id | id_domain |
notes | text | not null

psql:test.sql:9: ERROR: Existing attribute "id" cannot be a PRIMARY KEY
because it is not marked NOT NULL
---

Why NOT NULL modifier for ID_DOMAIN not present in the test table
definition?

Thanks