SQL works in 10g, not in 8i

This SQL works fine in 10g, but no in 8i. It stops at the subquery highlighting the SUM word, with the error: NOT a Group Expression.

What is wrong with it. How should I change it. Spent a few hours on it, and still can't figure it out. The rest of the query is fine. If I replace the subquery by 1, everything is okay.
The subquery by itself is also ok.

Can you help?
Thanks
Roberto

Select AS_OF_DATE, b.CD_ID,DEALNO ,SECID,

(PREM_DISC_AMOUNT/((EST_MDATE
-START_AMORT_DATE)+1)*((TO_DATE('31-jan-2005')+1)-START_AMORT_DATE)
-PREM_DISC_AMOUNT)

+ (SELECT sum(A.UNAMORT_PREM_DISC_BAL) FROM FASDIG.CALLABLE_DEAL A WHERE A.AS_OF_DATE = ADD_MONTHS('30-sep-2005',-2) AND A.SECID LIKE b.secid)
AS "AMORTIZATION CHECK" FROM FASDIG.CALLABLE_DEAL B WHERE AS_OF_DATE = '31-jan-2005' AND HEDGED_FLAG='Y' AND PREM_DISC_AMOUNT <>0 AND PREM_DISC_IND ='D'

GROUP BY
AS_OF_DATE, CD_ID, DEALNO , SECID, PREM_DISC_AMOUNT, EST_MDATE, START_AMORT_DATE