Nested Query Problem

I'm trying to create a query with a nested query and I get the dreaded "You
tried to execute a query that does not include the specific expression
FISYEBUD as part of an aggreagte function".

SELECT Data.FISYEBUD, Data.TRAID, Data.DOCLINNU, Data.PPROCODBUY, Data.APPR,
Sum(IIf(Data!ACCOTY=21 And Data![Reference Number] & Data!OCLINNUREF Is
Null,Data!AMTDOL,0)) AS [PO Amount], (SELECT Sum([Data]![AMTDOL]) From [Data]
As [OldData] Where [Old Data]![TRAID]=[Data]![Reference Number] And
[OldData]![DOCLINNU]=[Data]![OCLINNUREF]) AS Spent
FROM Data
GROUP BY Data.FISYEBUD, Data.TRAID, Data.DOCLINNU, Data.PPROCODBUY,
Data.APPR, Data.VENINVONU
HAVING (((Data.TRAID) Like "PO*") AND ((Data.VENINVONU)="2008-2009 flu
season"))
ORDER BY Data.TRAID, Data.DOCLINNU, Data.FISYEBUD;

I'm trying to query the amount of funds I have spent during the flu season.
--
JimT