Fikse 10-NULL=NULL

Jeg har et problem når der bliver regnet med NULL, eksempelvis bliver 10-NULL=NULL i stedet for 10-NULL=10, hvordan fikser man det i en query? Her er data og forespørgsel:

ORGANISATION
organisation_id | id | antal
222 | 10 | 5
222 | 20 | 2
333 | 10 | 2

STOCK
organisation_id | id | nummer
333 | 10 | 45312
333 | 10 | 45313
333 | 10 | 45314

FORESPØRGSEL
SELECT allocated, onstock, (allocated-onstock) AS to_be_used
FROM  (
    SELECT antal AS allocated, organisation_id
    FROM  organisation
    GROUP BY organisation_id, id
        ) a
LEFT JOIN
    (
    SELECT count(id) AS onstock, organisation_id
    FROM  stock
    GROUP BY id
    ) b
USING (organisation_id);


Giver noget i retning af:
allocated | onstock | to_be_used
5 | 3    | 2
2 | NULL | NULL
2 | NULL | NULL

??????