"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@oit.gatech.edu