Privileged API user cannot get members of a group

A privileged API user for a CO cannot query to find the members of a group in the CO.

An example curl command is

curl 

    -u 'co_18.rest_user_01:XXXXXXXX' \

    -X GET https://registry-test.cilogon.org/registry/co_group_members.json?cogroupid=248

This returns a 404.

I have verified that the username/password is correct (other API calls work) and that the CO Group with ID=248 is a group in the CO (it is a plain group).