list-disks action throws an exception on on bionic/train

This does not reproduce when I tried,

ubuntu@nkshirsagar-bastion:~/stsstack-bundles/ceph$ ./generate-bundle.sh -s bionic -r train -n bionic-train-ceph --run
Creating Juju model bionic-train-ceph
...
<snip>

ubuntu@nkshirsagar-bastion:~/stsstack-bundles/ceph$ juju run-action ceph-osd/leader list-disks --wait
unit-ceph-osd-0:
  UnitId: ceph-osd/0
  id: "2"
  results:
    blacklist: '[]'
    disks: '[''/dev/vdb'']'
    non-pristine: '[]'
  status: completed
  timing:
    completed: 2021-08-18 13:00:31 +0000 UTC
    enqueued: 2021-08-18 13:00:30 +0000 UTC
    started: 2021-08-18 13:00:31 +0000 UTC

/var/lib/juju/agents/unit-ceph-osd-0/charm/hooks/charmhelpers/core/host.py does have

def owner(path):
    """Returns a tuple containing the username & groupname owning the path.

    :param str path: the string path to retrieve the ownership
    :return tuple(str, str): A (username, groupname) tuple containing the
                             name of the user and group owning the path.
    :raises OSError: if the specified path does not exist
    """
    stat = os.stat(path)
    username = pwd.getpwuid(stat.st_uid)[0]
    groupname = grp.getgrgid(stat.st_gid)[0]
    return username, groupname