problem with subselect

Hi,

I have this :

SELECT date_trunc('month', f.date_creation) as mois,
sum(f.montant_ttc / d.taux) as facture,
(SELECT sum(f2.montant_ttc / d2.taux) as facture2
FROM facture AS f2 JOIN devise AS d2 USING (id_devise)
WHERE date_trunc('month', f2.date_creation)
<= date_trunc('month', f.date_creation))
FROM facture AS f JOIN devise AS d USING (id_devise)
GROUP BY date_trunc('month', f.date_creation)

which does not work... telling me that

"Sub-SELECT uses un-GROUPed attribute f.date_creation from outer query"
Which is not really true, but some way is...

What I want is to get the sum of the month, and the sum of the month and
the previous.

If someone has a clue, something, anything... :)

--
Mathieu Arnold