Do you see something wrong in my small change to this sql?

This sql is currently working. If you look at the 7th line, what I want now
it to use the idprt# which is their item number in a new version so the
user could select per which item(s) they wish to see. but when i simply,
take away the max it gives error when trying to run: I get error SQL 0122
IDPRT# in select is not valid. why is this, I just want to have that field
as a param.

SELECT "OEIND94"."IDDOCD" AS INV_DATE,
"OEIND94"."IDORD#" AS ORD_NUM,
"OEIND94"."IDORDT" AS ORD_TYPE,
"OEIND94"."IDPRLC" AS PROD_FAMILY,
"OEIND94"."IDPR$C" AS PRICE_CODE,
"OEIND94"."IDCOM#",
MAX("OEIND94"."IDPRT#") AS ITEM_REF,
"ICPRT1"."IARCC4" AS PROD_TYPE,
"ICPRT1"."IARC11" AS PROD_CLASS,
"ICPRT1"."IACMQC" AS CUM_CODE,
SUM("OEIND94"."IDSHP#") AS NUM_UNITS,
SUM("OEIND94"."IDSHP#" * "OEIND94"."IDCSU$") AS TOT_COST,
SUM("OEIND94"."IDSHP#" * "OEIND94"."IDNTU$") AS TOT_SLS
FROM ASTDTA"."OEIND94"
"OEIND94" INNER JOIN "ASTDTA"."ICPRT1"
"ICPRT1" ON "OEIND94"."IDPRT#"="ICPRT1"."IAPRT#"
WHERE "OEIND94"."IDCOM#"='001' AND
"OEIND94"."IDDOCD" >= {?FromDate} AND
"OEIND94"."IDDOCD" <= {?ToDate} AND
"ICPRT1"."IARCC4"='FIN'
GROUP BY "OEIND94"."IDDOCD",
"OEIND94"."IDORD#",
"OEIND94"."IDORDT",
"OEIND94"."IDPRLC",
"OEIND94"."IDPR$C",
"OEIND94"."IDCOM#",
"ICPRT1"."IARCC4" ,
"ICPRT1"."IARC11",
"ICPRT1"."IACMQC"
ORDER BY "OEIND94"."IDDOCD",
"OEIND94"."IDORD#",
"OEIND94"."IDORDT",
"OEIND94"."IDPRLC",
"OEIND94"."IDCOM#"