Error - n aggregate may not appear in the WHERE clause

Question

Hi

  On below code i am getting 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.

SELECT T1.ITEMCODE, T1.QTY, T2.QUANTITY FROM dbo.OW T0 INNER JOIN dbo.WO T1

ON T0.DOCENTRY = T1.DOCENTRY LEFT OUTER JOIN dbo.IG T2 ON T2.BASEENTRY = T0.DOCENTRY AND T1.ITEMCODE = T2.ITEMCODE 

WHERE T1.PLANNEDQTY > sum((ISNULL(T2.QUANTITY, 0))) AND T0.DOCENTRY = 1506

Thanks