plpgsql error: cache lookup from pg_proc failed

playpen=# create table aa(
playpen(# a int,
playpen(# b int,
playpen(# t timestamp
playpen(# );
CREATE
playpen=# 
playpen=# 
playpen=# CREATE FUNCTION touch () RETURNS OPAQUE AS '
playpen'#     BEGIN
playpen'#          new.t := current_timestamp;
playpen'#         RETURN new;
playpen'#     END;
playpen'# ' LANGUAGE 'plpgsql';
CREATE
playpen=# 
playpen=# 
playpen=# CREATE TRIGGER lastmod_1 AFTER update or insert ON aa FOR EACH
ROW EXECUTE PROCEDURE touch();
CREATE
playpen=# insert into aa (a, b) values (1,2);
ERROR:  plpgsql: cache lookup from pg_proc failed

What does this error message mean?

-- 
Joseph Shraibman
jks@selectacast.net
Increase signal to noise ratio.  http://www.targabot.com