Referencing to system catalog problem

I am logged in as superuser. I am trying to create something similar to
this:

Code:
CREATE TABLE tbl_unit_convfunctions(
unit_from integer REFERENCES tbl_units (unit_id),
unit_to integer REFERENCES tbl_units (unit_id),
proc_id oid REFERENCES pg_proc (oid)
)but no matter what I refer to from pg_proc, i get the error message:

ERROR: permission denied: "pg_proc" is a system catalog
SQL state: 42501

Has anyone any suggestions how to do something similar, or even better: how
to solve this error. I couldn't find any useful information on the net about
this issue.

Thanks,
Davor

Original post:
http://forums.devshed.com/postgresql-help-21/referencing-to-system-catalog-problem-670063.html

Note: OID's are unique in that table, and should be referable, and I
explicitely granted the REFERENCE priviledge to the superuser.