'' != NULL in PostGres???

Hello. I am experimenting with Postgresql-6.3.2 on a RedHat 5.1 DEC Alpha:

nuacct=> create table users (
nuacct-> recno int,
nuacct-> custid int unique,
nuacct-> username char(14) not null,
nuacct-> service int
nuacct-> );
NOTICE: CREATE TABLE/UNIQUE will create implicit index users_custid_key
for table users
CREATE

nuacct=> insert into users values
nuacct-> ( 0, 0, '', 0 );
INSERT 18241 1

??? Shouldn't that have failed, generating an error ???

nuacct=> select * from users where username is null;
recno|custid|username|service
-----+------+--------+-------
(0 rows)

nuacct=> select * from users where username = '';
recno|custid| username|service
-----+------+--------------+-------
0| 0| | 0
(1 row)

nuacct=> \d users
Table = users
+----------------------------------+------------------------------+------+
| Field | Type |Length|
+----------------------------------+------------------------------+------+
| recno | int4 |4 |
| custid | int4 |4 |
| username | char() not null |14 |
| service | int4 |4 |
+----------------------------------+------------------------------+------+

???