1.9 rc2: error in migration when migrating unmanaged subnet

The following traceback was seen while migrating from MAAS 1.8.3 to MAAS 1.9 rc2:

Error in migration: maasserver:0146_populate_subnets
Traceback (most recent call last):
  File "/usr/sbin/maas-region-admin", line 68, in <module>
    main()
  File "/usr/sbin/maas-region-admin", line 64, in main
    run()
  File "/usr/sbin/maas-region-admin", line 57, in run
    management.execute_from_command_line()
  File "/usr/lib/python2.7/dist-packages/django/core/management/__init__.py", line 399, in execute_from_command_line
    utility.execute()
  File "/usr/lib/python2.7/dist-packages/django/core/management/__init__.py", line 392, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/lib/python2.7/dist-packages/django/core/management/base.py", line 242, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/usr/lib/python2.7/dist-packages/django/core/management/base.py", line 285, in execute
    output = self.handle(*args, **options)
  File "/usr/lib/python2.7/dist-packages/south/management/commands/migrate.py", line 107, in handle
    ignore_ghosts = ignore_ghosts,
  File "/usr/lib/python2.7/dist-packages/south/migration/__init__.py", line 219, in migrate_app
    success = migrator.migrate_many(target, workplan, database)
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 235, in migrate_many
    result = migrator.__class__.migrate_many(migrator, target, migrations, database)
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 310, in migrate_many
    result = self.migrate(migration, database)
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 133, in migrate
    result = self.run(migration)
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 107, in run
    return self.run_migration(migration)
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 81, in run_migration
    migration_function()
  File "/usr/lib/python2.7/dist-packages/south/migration/migrators.py", line 57, in <lambda>
    return (lambda: direction(orm))
  File "/usr/lib/python2.7/dist-packages/maasserver/migrations/0146_populate_subnets.py", line 31, in forwards
    now, NodeGroupInterface, Subnet, default_space, default_vlan)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/migrations/populate_subnets_helper.py", line 120, in _migrate_nodegroupinterfaces_forward
    now, ngi, Subnet, default_space, default_vlan)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/migrations/populate_subnets_helper.py", line 137, in _create_subnet_from_nodegroupinterface
    subnet.save()
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 545, in save
    force_update=force_update, update_fields=update_fields)
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 573, in save_base
    updated = self._save_table(raw, cls, force_insert, force_update, using, update_fields)
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 654, in _save_table
    result = self._do_insert(cls._base_manager, using, fields, update_pk, raw)
  File "/usr/lib/python2.7/dist-packages/django/db/models/base.py", line 687, in _do_insert
    using=using, raw=raw)
  File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 232, in _insert
    return insert_query(self.model, objs, fields, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 1511, in insert_query
    return query.get_compiler(using=using).execute_sql(return_id)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 897, in execute_sql
    for sql, params in self.as_sql():
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/compiler.py", line 855, in as_sql
    for obj in self.query.objs
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 350, in get_db_prep_save
    prepared=False)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/__init__.py", line 342, in get_db_prep_value
    value = self.get_prep_value(value)
  File "/usr/lib/python2.7/dist-packages/maasserver/fields.py", line 592, in get_prep_value
    return parse_cidr(value)
  File "/usr/lib/python2.7/dist-packages/maasserver/fields.py", line 578, in parse_cidr
    return unicode(IPNetwork(value).cidr)
  File "/usr/lib/python2.7/dist-packages/netaddr/ip/__init__.py", line 907, in __init__
    implicit_prefix, flags)
  File "/usr/lib/python2.7/dist-packages/netaddr/ip/__init__.py", line 802, in parse_ip_network
    raise TypeError('unexpected type %s for addr arg' % type(addr))
TypeError: unexpected type <type 'NoneType'> for addr arg