ERROR (Bug?) in RULE processing ?

Can somebody tell what this errormessage means ?
ERROR: fireRIRrules: failed to remove aggs from qual

I don't know what "aggs" and "qual" is.

What I did:
arbeitsdb=# select id,type,subtype,name from documents;
id | type | subtype | name
----+------+---------+---------------
1 | 1 | 5 | motiv_el_01
2 | 1 | 1 | def_el_01
3 | 2 | 10 | deduc_el_01
4 | 2 | 18 | example_el_01
(4 rows)

arbeitsdb=# insert into documents(type,subtype,name)
arbeitsdb-# values (2,16,'foo');
INSERT 55505 1
arbeitsdb=# delete from documents where oid=55505;
DELETE 1
arbeitsdb=# CREATE RULE doc_contained AS ON INSERT TO documents WHERE
arbeitsdb-# ((NEW.type = 1) AND (count( (SELECT element_id
arbeitsdb(# FROM element_contained_in WHERE (element_id =
currval('docs_seq')) )) = 0))
arbeitsdb-# DO INSTEAD NOTHING;
CREATE
arbeitsdb=# insert into documents(type,subtype,name)
arbeitsdb-# values (2,16,'foo');
ERROR: fireRIRrules: failed to remove aggs from qual

Thanks Fritz