How to set where condition for aggregate function.

Question

select SUM(dbo.GLAFS.OPENBAL) + SUM(dbo.GLAFS.NETPERD1) AS January from GLAFS where SUM(dbo.GLAFS.OPENBAL) + SUM(dbo.GLAFS.NETPERD1) > 0

its giving an error "

An aggregate may not appear in the WHERE clause unless it is in a subquery contained in a HAVING clause or a select list, and the column being aggregated is an outer reference" 

 

How can i resolve this issue. Please help.