group by problem

Moin,

i have an sql query which works perfect under PostgrSQL
6.0 but under 6.5.1 it brings: ERROR: Illegal use of
aggregates or non-group column in target list.

select T1.id, T1.name, T1.zusatz, T1.kontakt, T1.strasse,
T1.land, T1.plz, T1.ort, T1.telefax
from debitoren T1, auftrag T2
where T2.kunde=T1.id
group by T1.id;

Where is the problem?

Thanks for any hints

Bis dann
Matthias