Problem when group by a sub-query
I have
select (select cobrador from BUSCA_COBRADOR_POR_aplicante (a.clave_fact)),a.ftot
from VISTA_COMPROBANTES a

(BUSCA_COBRADOR_POR_aplicante is a Stored Procedure)

Result
Cobrador Ftot
------- ----
1 369.00
3 122.00
1 72.00
11 310.00
1 85.00
3 560.00

I must sum(ftot) group by cobrador, like this

Cobrador Sum
-------- ---
1 526.00
3 682.00
11 310.00

But in Firebird 2.1.6.18518, this sql show an error
select (select cobrador from BUSCA_COBRADOR_POR_aplicante
(a.clave_fact))as cobrador,sum(a.ftot)
from VISTA_COMPROBANTES a
group by cobrador

Invalid expression in the select list (not contained in either an
aggregate function or the GROUP BY clause)

Any workarround ?
Regards