problem with keyword 'old'

I am ussing Postgresql v. 6.3 in a RedHat Linux system. I try to create a
rule to log the deletes on table 'filter' ussing the following:

CREATE RULE log_delete_filter AS ON DELETE TO filter
DO
INSERT INTO log_activity (mod_table, mod_record, mod_type)
VALUES ('filter', old.did, 'D');

The atributes of tables are:
filter (did int4, text text)
log_activity (mod_table char(20), mod_rec int4, mod_type char(1) )

I have the following result:
ERROR: old: Table does not exist.
and the rule is not created.

I tested a same rule on INSERT with the keyword new.did and is working
perfect.
Also I tested the commands:
SELECT new.did;
> ERROR: NEW used in non-rule query
SELECT current.did;
>ERROR: CURRENT used in non-rule query
SELECT old.did;
>ERROR: old: Table does not exist.

What is going on? Do you have any idea why does system not undestand
keyword 'old'?

Sincerely,
Basilis Kladis <bkladis(at)knowledge(dot)gr>

Βασίλης Κλάδης
Language Engineering Dept.
KNOWLEDGE S.A.