Comment 1 for bug 1721200

It looks like there is a specific code path for a subordinate to find out information about the principle for juju 1.X:

def principal_unit():
    """Returns the principal unit of this unit, otherwise None"""
    # Juju 2.2 and above provides JUJU_PRINCIPAL_UNIT
    principal_unit = os.environ.get('JUJU_PRINCIPAL_UNIT', None)
    # If it's empty, then this unit is the principal
    if principal_unit == '':
        return os.environ['JUJU_UNIT_NAME']
    elif principal_unit is not None:
        return principal_unit
    # For Juju 2.1 and below, let's try work out the principle unit by
    # the various charms' metadata.yaml.
    for reltype in relation_types():
        for rid in relation_ids(reltype):
            for unit in related_units(rid):
                md = _metadata_unit(unit)
                if not md:
                    continue
                subordinate = md.pop('subordinate', None)
                if not subordinate:
                    return unit
    return None

It looks like the principle is returning invalid yaml.