Comment 11 for bug 234727

getent group admin|cut -d: -f4|cut -d, -f1
On my system, returns 'instuser'
on my home system, returns 'tom'

This is the first user who set up the system, and this script automatically assumes that they're the first valid user in the admin group. I'm seeing this sudo error on systems where the local installation user, ie instuser; exists in a local context only, or not at all, and userdel has failed to remove them from the system.
that getent group command might not always return a valid user with a valid home directory.