Add fixed IP fails

I try to add a fixed IP to instance with this command: 'nova add-fixed-ip 4 2' but I get this error in nova-network logs:

2011-08-30 16:00:27,111 DEBUG nova.rpc [-] {u'_context_roles': [u'Admin'], u'_context_request_id': u'3d96c679-86f8-49c2-b7ba-9d31f0b76ebc', u'_context_read_deleted': False, u'args': {u'instance_id': 4, u'network_id': u'2', u'host': u'p-hs21-16'}, u'_context_auth_token': u'999888777666', u'_context_is_admin': None, u'_context_project_id': u'admin', u'_context_timestamp': u'2011-08-30T14:00:26.869559', u'_context_user_id': u'admin', u'method': u'add_fixed_ip_to_instance', u'_context_remote_address': u'127.0.0.1'} reçu from (pid=5430) process_data /usr/lib/pymodules/python2.7/nova/rpc/amqp.py:200
2011-08-30 16:00:27,111 DEBUG nova.rpc [-] Contexte décompacté : {'user_id': u'admin', 'roles': [u'Admin'], 'timestamp': u'2011-08-30T14:00:26.869559', 'auth_token': u'999888777666', 'msg_id': None, 'remote_address': u'127.0.0.1', 'is_admin': None, 'request_id': u'3d96c679-86f8-49c2-b7ba-9d31f0b76ebc', 'project_id': u'admin', 'read_deleted': False} from (pid=5430) _unpack_context /usr/lib/pymodules/python2.7/nova/rpc/amqp.py:432
2011-08-30 16:00:27,117 DEBUG nova.network.manager [-] kwargs: {} from (pid=5430) _allocate_fixed_ips /usr/lib/pymodules/python2.7/nova/network/manager.py:131
2011-08-30 16:00:27,117 ERROR nova [-] Exception during message handling
(nova): TRACE: Traceback (most recent call last):
(nova): TRACE: File "/usr/lib/pymodules/python2.7/nova/rpc/amqp.py", line 230, in _process_data
(nova): TRACE: rval = node_func(context=ctxt, **node_args)
(nova): TRACE: File "/usr/lib/pymodules/python2.7/nova/network/manager.py", line 570, in add_fixed_ip_to_instance
(nova): TRACE: self._allocate_fixed_ips(context, instance_id, host, networks)
(nova): TRACE: File "/usr/lib/pymodules/python2.7/nova/network/manager.py", line 132, in _allocate_fixed_ips
(nova): TRACE: vpn = kwargs.pop('vpn')
(nova): TRACE: KeyError: 'vpn'
(nova): TRACE:

I use nova bzr 1508 in VLAN network.