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