'' != 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     |
+----------------------------------+------------------------------+------+

???