referential integrity violation

Hi guys

I have created a Pg DB, Ive included 2 of the tables.
When inserting non NULL values everything works just fine,
but when i want to leave a table empty i get the following error:

EXEC: ERROR: referential integrity violation - key referenced from pc not found in slip_printer

Im recieving the information from a Perl CGI form that is filled in and then INSERT it using perl DBI

If any more information is needed please let me know. As far as i know i should be allowed to have null.

TIA
Merritt

CREATE TABLE slip_printer (
id SERIAL PRIMARY KEY,
make VARCHAR(256) NOT NULL,
model VARCHAR(256) NOT NULL,
UNIQUE (make, model)
);

CREATE TABLE pc (
id SERIAL PRIMARY KEY,
store INTEGER REFERENCES store (id) NOT NULL,
cpu_type INTEGER REFERENCES cpu_type (id) NOT NULL,
cpu_mhz INTEGER REFERENCES cpu_mhz (id) NOT NULL,
memory_type INTEGER REFERENCES memory_type (id) NOT NULL,
memory_amount INTEGER, /* amount of chips/sticks, not size in MB */
hdd INTEGER REFERENCES hdd (id),
hdd_serial VARCHAR(256),
monitor INTEGER REFERENCES monitor (id) NOT NULL,
slip_printer INTEGER REFERENCES slip_printer (id),
report_printer INTEGER REFERENCES report_printer (id),
cash_drawer INTEGER REFERENCES cash_drawer (id) NOT NULL,
scanner INTEGER REFERENCES scanner (id) NOT NULL,
ups INTEGER REFERENCES ups (id),
modem INTEGER REFERENCES modem (id)
);