using count and odd "group by" errors...

I've been reading the Informix SQL manual for a while and can't seem to find an explanation which clears up an issue I have regarding the use of "count." I'm looking for a way to select rows which have duplicate values of a non-key field. If I have a "people" table keyed on person_id, I would like to display all people who share a "nickname" with other people. Hopefully grouped and ordered by nickname. I tried: SELECT COUNT(nickname), nickname FROM people GROUP BY nickname HAVING COUNT(nickname) > 1 ORDER BY 1 DESC which gives me a nice list of the most frequently used nicknames in descending order (and the count, of course). But when I change the select statement to: SELECT COUNT(nickname), nickname, person_id I get an error stating that person_id needs to be added to the group by list. Then when I do add it, I get no results! I realize that I'm not grasping something important here. Can someone tell me what it is?