Crystal Reports group error on SUM command

Message

     I have the following line sample:
sum ({REPORT_BASE.PT_S},{REPORT_BASE.PROD_ID});

This formula is on a group footer line
PT_S is a numeric field
where PROD_ID is a group in  a header and footer

I get error msg:
While processing reportname.rpt report
Crystal Reports returned -2147191779
There must be a group that matches this field

There is no such CryRep error number that I can find
I have a valid group, a valid database field, but can't get past this error

Any wisdom will be much appreciated ... Wally