katello-backup fails on Red Hat Capsule with error: can't find group for postgres.

Issue

• Capturing katello-backup on Capsule fails with below given error:

[root@capsule /]# katello-backup --online-backup /katello_bkp
Starting backup: 2018-11-16 13:53:47 +0000
Creating backup folder /katello_bkp/katello-backup-20181116135347
/usr/share/ruby/fileutils.rb:1102:in `getgrnam': can't find group for postgres (ArgumentError)
        from /usr/share/ruby/fileutils.rb:1102:in `fu_get_gid'
        from /usr/share/ruby/fileutils.rb:1062:in `chown_R'
        from /bin/katello-backup:189:in `create_directories'
        from /bin/katello-backup:306:in `<main>'

• Why am I able to take a katello-backup on Satellite but the same fails on Capsule?