group by error message?

Hi,

Running this query:

    critik=# select distinct pt.type_fr,sum(e2.id_event) from person_type pt natural join person_to_event join event e
using(id_event) LEFT JOIN event e2 ON e.id_event = e2.id_event AND e2.id_event=219 join event_type et ON
e.id_event_type= et.id_event_type where et.type_fr='théâtre' GROUP BY pt.type_fr IS NULL; 

gives this error:

    ERROR:  column "pt.type_fr" must appear in the GROUP BY clause or be used in an aggregate function

But pt.type_fr is _already_ in the group by.

When adding e2.id_event however ("GROUP BY pt.type_fr,e2.id_event") it works.

Did I miss something there?