Error when editing user

I created a new user named tester2, then tried to add that user to the wiki group.

[2016-05-01 17:56:09,071] plinth.actions ERROR    Error executing command - ['sudo', '-n', '/usr/share/plinth/actions/ssh', 'set-keys', '--username', 'tester2', '--keys', ''], , Traceback (most recent call last):
  File "/usr/share/plinth/actions/ssh", line 99, in <module>
    main()
  File "/usr/share/plinth/actions/ssh", line 95, in main
    subcommand_method(arguments)
  File "/usr/share/plinth/actions/ssh", line 73, in subcommand_set_keys
    subprocess.check_call(['mkhomedir_helper', user])
  File "/usr/lib/python3.5/subprocess.py", line 581, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['mkhomedir_helper', 'tester2']' returned non-zero exit status 8