'FlatManager' object has no attribute 'allocate_floating_ip'

I am now using the FlatManager network mode, when I allocate a floating ip to a instance with nova api command:

nova floating-ip-create

I get the error log in the server side:

nova-api log:
2012-04-27 20:02:44 TRACE nova.api.openstack File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/rpc/__init__.py", line 68, in call
2012-04-27 20:02:44 TRACE nova.api.openstack return _get_impl().call(context, topic, msg, timeout)
2012-04-27 20:02:44 TRACE nova.api.openstack File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/rpc/impl_kombu.py", line 674, in call
2012-04-27 20:02:44 TRACE nova.api.openstack return rpc_amqp.call(context, topic, msg, timeout, Connection.pool)
2012-04-27 20:02:44 TRACE nova.api.openstack File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/rpc/amqp.py", line 338, in call
2012-04-27 20:02:44 TRACE nova.api.openstack rv = list(rv)
2012-04-27 20:02:44 TRACE nova.api.openstack File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/rpc/amqp.py", line 306, in __iter__
2012-04-27 20:02:44 TRACE nova.api.openstack raise result
2012-04-27 20:02:44 TRACE nova.api.openstack RemoteError: Remote error: AttributeError 'FlatManager' object has no attribute 'allocate_floating_ip'

nova-network log:
2012-04-27 20:02:44 TRACE nova.rpc.amqp Traceback (most recent call last):
2012-04-27 20:02:44 TRACE nova.rpc.amqp File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/rpc/amqp.py", line 249, in _process_data
2012-04-27 20:02:44 TRACE nova.rpc.amqp node_func = getattr(self.proxy, str(method))
2012-04-27 20:02:44 TRACE nova.rpc.amqp File "/usr/lib/python2.6/site-packages/nova-2012.1-py2.6.egg/nova/service.py", line 213, in __getattr__
2012-04-27 20:02:44 TRACE nova.rpc.amqp return getattr(manager, key)
2012-04-27 20:02:44 TRACE nova.rpc.amqp AttributeError: 'FlatManager' object has no attribute 'allocate_floating_ip'

It is ok if I change the network mode to FlatDHCPManager. I read the nova source code and found that FloatingIP is the only class that have the "allocate_floating_ip" method and FlatDHCPManager is a subclass of this class， but class FlatManager is not a subclass of this class:

Code piece from network/manager.py:
class FlatDHCPManager(RPCAllocateFixedIP, FloatingIP, NetworkManager):

class FlatManager(NetworkManager):

class VlanManager(RPCAllocateFixedIP, FloatingIP, NetworkManager):