AttributeError in VlanManager.periodic_tasks

When running nova-network, the periodic task execution fails with an AttributeError, because the wrong value has been passed into db.fixed_ip_disassociate_all_by_timeout:

2010-10-02 12:44:56+0100 [-] Unhandled Error
        Traceback (most recent call last):
          File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 944, in unwindGenerator
            return _inlineCallbacks(None, f(*args, **kwargs), Deferred())
          File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 823, in _inlineCallbacks
            result = g.send(result)
          File "/home/emellor/openstack/nova/xenapi/nova/service.py", line 149, in periodic_tasks
            yield self.manager.periodic_tasks(context)
          File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 944, in unwindGenerator
            return _inlineCallbacks(None, f(*args, **kwargs), Deferred())
        --- <exception caught here> ---
          File "/usr/lib/python2.6/dist-packages/twisted/internet/defer.py", line 823, in _inlineCallbacks
            result = g.send(result)
          File "/home/emellor/openstack/nova/xenapi/nova/network/manager.py", line 235, in periodic_tasks
            time)
          File "/home/emellor/openstack/nova/xenapi/nova/db/api.py", line 209, in fixed_ip_disassociate_all_by_timeout
            return IMPL.fixed_ip_disassociate_all_by_timeout(context, host, time)
          File "/home/emellor/openstack/nova/xenapi/nova/db/sqlalchemy/api.py", line 90, in wrapper
            if not is_admin_context(args[0]):
          File "/home/emellor/openstack/nova/xenapi/nova/db/sqlalchemy/api.py", line 44, in is_admin_context
            return context.is_admin
        exceptions.AttributeError: 'VlanManager' object has no attribute 'is_admin'