SQL 2005 Reporting Services - Aggregate Sum function with an IIf function

SELECT     Office.OffName, SkillLevel.SkDesc, Employee.Emplname, Employee.Empfname, Employee.Empnum, SUM(IIf(WIP.Wbillable=1, WIP.Whours, NULL)) AS BillableHours
FROM         Employee INNER JOIN
                      Office ON Employee.Empoff = Office.OffID INNER JOIN
                      SkillLevel ON Employee.Emplevel = SkillLevel.SkID RIGHT OUTER JOIN
                      WIP ON Employee.ID = WIP.WempID
GROUP BY Office.OffName, Employee.Empnum, Employee.Emplname, Employee.Empfname, SkillLevel.SkDesc
ORDER BY Office.OffName, SkillLevel.SkDesc