diff --git a/callcontrol/rating/backends/sipthor.py b/callcontrol/rating/backends/sipthor.py
index 0bab5a4..5e4b965 100644
--- a/callcontrol/rating/backends/sipthor.py
+++ b/callcontrol/rating/backends/sipthor.py
@@ -1,129 +1,129 @@
 
 from application.version import Version
 from application.configuration import ConfigSection, ConfigSetting
 from application.system import host
 from application import log
 from application.python.types import Singleton
 
 from gnutls.interfaces.twisted import TLSContext, X509Credentials
 
 from thor import __version__ as thor_version
 from thor.eventservice import EventServiceClient, ThorEvent
 from thor.entities import ThorEntitiesRoleMap, GenericThorEntity as ThorEntity
 
 from callcontrol.tls import Certificate, PrivateKey
 from callcontrol.rating import RatingEngine, RatingEngineAddress
 from callcontrol import configuration_filename, __version__
 
 from twisted.internet import defer, reactor
 
-if Version.parse(thor_version) < '1.1.21':
+if Version.parse(thor_version) < Version.parse('1.1.21'):
     raise RuntimeError('Thor version is smaller the 1.1.21 (%s)' % thor_version)
 
 
 class ThorNodeConfig(ConfigSection):
     __cfgfile__ = configuration_filename
     __section__ = 'ThorNetwork'
 
     enabled = False
     domain = "sipthor.net"
     multiply = 1000
     certificate = ConfigSetting(type=Certificate, value=None)
     private_key = ConfigSetting(type=PrivateKey, value=None)
     ca = ConfigSetting(type=Certificate, value=None)
 
 
 class CallcontrolNode(EventServiceClient):
     __metaclass__ = Singleton
     topics = ["Thor.Members"]
 
     def __init__(self):
         self.node = ThorEntity(host.default_ip, ['call_control'], version=__version__)
         self.networks = {}
         self.rating_connections = {}
         self.presence_message = ThorEvent('Thor.Presence', self.node.id)
         self.shutdown_message = ThorEvent('Thor.Leave', self.node.id)
         credentials = X509Credentials(ThorNodeConfig.certificate, ThorNodeConfig.private_key, [ThorNodeConfig.ca])
         credentials.verify_peer = True
         tls_context = TLSContext(credentials)
         EventServiceClient.__init__(self, ThorNodeConfig.domain, tls_context)
 
     def publish(self, event):
         self._publish(event)
 
     def stop(self):
         return self._shutdown()
 
     def connectionLost(self, connector, reason):
         """Called when an event server connection goes away"""
         self.connections.discard(connector.transport)
 
     def connectionFailed(self, connector, reason):
         """Called when an event server connection has an unrecoverable error"""
         connector.failed = True
 
     def _disconnect_all(self, result):
         for conn in self.connectors:
             conn.disconnect()
 
     def _shutdown(self):
         if self.disconnecting:
             return
         self.disconnecting = True
         self.dns_monitor.cancel()
         if self.advertiser:
             self.advertiser.cancel()
         if self.shutdown_message:
             self._publish(self.shutdown_message)
         requests = [conn.protocol.unsubscribe(*self.topics) for conn in self.connections]
         d = defer.DeferredList([request.deferred for request in requests])
         d.addCallback(self._disconnect_all)
         return d
 
     def handle_event(self, event):
         reactor.callFromThread(self._handle_event, event)
 
     def _handle_event(self, event):
         networks = self.networks
         role_map = ThorEntitiesRoleMap(event.message)  # mapping between role names and lists of nodes with that role
         role = 'rating_server'
         try:
             network = networks[role]
         except KeyError:
             from thor import network as thor_network
             network = thor_network.new(ThorNodeConfig.multiply)
             networks[role] = network
         new_nodes = set([node.ip for node in role_map.get(role, [])])
         old_nodes = set(network.nodes)
         added_nodes = new_nodes - old_nodes
         removed_nodes = old_nodes - new_nodes
         if added_nodes:
             for node in added_nodes:
                 network.add_node(node)
                 address = RatingEngineAddress(node)
                 self.rating_connections[address] = RatingEngine(address)
             plural = 's' if len(added_nodes) != 1 else ''
             log.msg("Added rating node%s: %s" % (plural, ', '.join(added_nodes)))
         if removed_nodes:
             for node in removed_nodes:
                 network.remove_node(node)
                 address = RatingEngineAddress(node)
                 self.rating_connections[address].shutdown()
                 del self.rating_connections[address]
             plural = 's' if len(removed_nodes) != 1 else ''
             log.msg("Removed rating node%s: %s" % (plural, ', '.join(removed_nodes)))
 
 
 class SipthorBackend(object):
 
     def __init__(self):
         self.node = CallcontrolNode()
 
     @property
     def connections(self):
         return self.node.rating_connections.values()
 
     def shutdown(self):
         for connection in self.connections:
             connection.shutdown()
         return self.node.stop()