"ON INSERT rule may not use OLD" on simple query

Postgresql 7.3.2 gives the error "ON INSERT rule may not use OLD" when
trying to define the following rule:

CREATE TABLE numbers ( number INTEGER );
CREATE TABLE other_numbers ( number INTEGER );

CREATE OR REPLACE RULE number_rule AS
ON INSERT TO numbers
DO INSERT INTO other_numbers
SELECT 1 EXCEPT SELECT 2;

although the rule, even upon close inspection, seems to make no
reference at all to the OLD pseudo-relation. Given the above table
definitions, the query:

SELECT 1 EXCEPT SELECT 2;

by itself runs fine and returns 1 as expected. (We have tested this
against both the unstable Debian package for 7.3.2, and against a copy
compiled locally using the standard source archive.)

--
Brandon Craig Rhodes http://www.rhodesmill.org/brandon
Georgia Tech brandon(at)oit(dot)gatech(dot)edu