Copying null values

Hi all,
I am using:

Debian 2.2 (kernel 2.2.18)
PgSQL version 7.0.3


I would like empty fields from a file being recognised as NULL values.
It doesn't seem to work when a COPY is done, but when I make an INSERT
without
specifying the value, this is recognised as a NULL

The file I want to copy is:

,0,Unknown,01/01/3000,01/01/3000,01/01/3000,0,
1,3,,01/01/3000,01/01/3000,01/01/3000,0,
2,,Unknown,01/01/3000,01/01/3000,01/01/3000,0,

What I am doing is:

--------------------------------------------------------------------------

CREATE TABLE bill(bill_id  SERIAL PRIMARY KEY,code  INTEGER,bill_number CHAR(20),issue_date DATE DEFAULT CURRENT_DATE
NOTNULL,expire_date DATE,negotiate_date DATE,amount  DECIMAL(9,2) NOT NULL,comment  TEXT
 
);
CREATE FUNCTION bill_id_max()
RETURNS INT AS 'SELECT max(bill_id) FROM bill'
LANGUAGE 'sql';

COPY bill FROM '/home/amanda/informatica/provas/bills-copy.dat' USING
DELIMITERS ',' WITH NULL AS '\,';
SELECT setval('bill_bill_id_seq',bill_id_max());
INSERT INTO bill (code,amount) VALUES (34,100000);
INSERT INTO bill (bill_number,amount) VALUES ('494949/949',100000);
SELECT * FROM bill WHERE bill_number IS NULL;
SELECT * FROM bill WHERE code IS NULL;

COPY bill TO '/tmp/copytest.out' USING DELIMITERS ',';

----------------------------------------------------------------------------

The result is:
setval
--------     2
(1 row)

INSERT 29322 1
INSERT 29323 1bill_id | code | bill_number | issue_date | expire_date |
negotiate_date | amount   | comment
---------+------+-------------+------------+-------------+----------------+-----------+---------
      3 |   34 |             | 2001-04-10 |
|                | 100000.00 |
(1 row)
bill_id | code |     bill_number      | issue_date | expire_date |
negotiate_date |  amount   | comment
---------+------+----------------------+------------+-------------+----------------+-----------+---------
      4 |      | 494949/949           | 2001-04-10 |
|                | 100000.00 |
(1 row)

COPY

This is the result file /tmp/copytest.out

0,0,Unknown             ,3000-01-01,3000-01-01,3000-01-01,0.00,
1,3,                    ,3000-01-01,3000-01-01,3000-01-01,0.00,
2,0,Unknown             ,3000-01-01,3000-01-01,3000-01-01,0.00,
3,34,\N,2001-04-10,\N,\N,100000.00,\N
4,\N,494949/949          ,2001-04-10,\N,\N,100000.00,\N

   Thanks