Problems with case statement in group by clause

I am having problems using select case to selectively choose a value that will be used in a group-by clause. The SQL works without the case. I am able to use a case in a select alone, without a group-by. This is a UDB database.

Been banging my head on this for a few days, any help would be greatly appreciated:

The following SQL returns the error:
"SQL0206N "ALLOC_PCT" is not valid in the context where it is used.
SQLSTATE=42703"


SELECT DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C,
DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT,
DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C,
Count(DBBLTRN.FITBT231_BR_HRDWR.CMPTR_NA) AS Count,
case when DBBLTRN.FITBT121_BLTO_SBU.BILL_ENTTY_C is null
then 1
else DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT
end
AS Alloc_Pct
FROM ((DBBLTRN.FITBT021_BLTO_ENT
RIGHT JOIN DBBLTRN.FITBT231_BR_HRDWR
ON DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C = DBBLTRN.FITBT231_BR_HRDWR.BILL_ENTTY_C)
LEFT JOIN DBBLTRN.FITBT121_BLTO_SBU
ON DBBLTRN.FITBT231_BR_HRDWR.BILL_ENTTY_C = DBBLTRN.FITBT121_BLTO_SBU.BILL_ENTTY_C)
LEFT JOIN DBBLTRN.FITBT171_CHGTYINP
ON (DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO = DBBLTRN.FITBT171_CHGTYINP.RPT_Y_MO)
AND (DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C = DBBLTRN.FITBT171_CHGTYINP.CHRG_TY_C)
GROUP BY DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C,
DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT,
DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C,
Alloc_Pct

---------------------------------------------------------

This SQL works:
SELECT DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C,
DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT,
DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C,
Count(DBBLTRN.FITBT231_BR_HRDWR.CMPTR_NA) AS Count,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT
FROM ((DBBLTRN.FITBT021_BLTO_ENT
RIGHT JOIN DBBLTRN.FITBT231_BR_HRDWR
ON DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C = DBBLTRN.FITBT231_BR_HRDWR.BILL_ENTTY_C)
LEFT JOIN DBBLTRN.FITBT121_BLTO_SBU
ON DBBLTRN.FITBT231_BR_HRDWR.BILL_ENTTY_C = DBBLTRN.FITBT121_BLTO_SBU.BILL_ENTTY_C)
LEFT JOIN DBBLTRN.FITBT171_CHGTYINP
ON (DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO = DBBLTRN.FITBT171_CHGTYINP.RPT_Y_MO)
AND (DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C = DBBLTRN.FITBT171_CHGTYINP.CHRG_TY_C)
GROUP BY DBBLTRN.FITBT021_BLTO_ENT.BILL_ENTTY_C,
DBBLTRN.FITBT231_BR_HRDWR.RPT_Y_MO,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT,
DBBLTRN.FITBT231_BR_HRDWR.CHRG_TY_C,
DBBLTRN.FITBT121_BLTO_SBU.BILL_TO_ENTTY_CHRG_PT