referencing a sytem table

hi,

environment: PostgreSQL 7.0.3,i686-pc-linux-gnu

to save more data of my database users i wanted to make a user_profile
table wihich is referenced to pg shadow 1:1. trying this i encountered the
problem that postgres does not semm to force referential integrity when
referencing a system table. is that right - or am i wrong? how can i do a
thing like this?

here are the sql statements i used:

Create Table profile_test (
p_usename name)

Alter table profile_test add constraint fk_1 FOREIGN KEY (p_usename)
REFERENCES
pg_shadow(usename) MATCH FULL;

insert into profile_test (p_usename) VALUES ('nosuchuser') ----> SUCCESS!!!!

thanks in advance
clemens