SUMIF clause not working in Groupby Query (neither CASE or IF) with Query Tables

SUMIF clause not working in Groupby Query (neither CASE or IF) with Query Tables

SELECT
         FACT."CNT.Nombre de Cuenta" Cliente, 
         sumif(FACT."FACT.Naturaleza ingresos" = 'Facturación Proyectos Ajenos', FACT."FACT.Subtotal", FACT."FACT.Subtotal") IngresosProA  
FROM  "BR-Facturas-2016" FACT 
GROUP BY  FACT."CNT.Nombre de Cuenta" 

Yields error:
nsupported function used in the query. Please send us your query to support@zohoreports.com, incase you wish to post to our bugs database