Internal Server Error when saving a cluster without Router IP

When saving a cluster DHCP configuration without Router IP, Internal Server Error is shown in the WebUI:

==> /var/log/maas/regiond.log <==
2015-04-08 21:53:41 [-] 127.0.0.1 - - [09/Apr/2015:01:53:40 +0000] "GET /MAAS/rpc/ HTTP/1.1" 200 166 "-" "Twisted PageGetter"
2015-04-08 21:53:52 [maas] WARNING: Failed to create Network when adding/editing cluster interface maas-virbr0 with error [{'ip': [u'Network with this Ip already exists.'], 'name': [u'Network with this Name already exists.']}]. This is OK if it already exists.
2015-04-08 21:53:52 [HTTPChannel,0,127.0.0.1] 500 Error - /MAAS/clusters/0a802d12-ad44-4b26-8f8d-9665b47a49ff/interfaces/virbr0/edit/
 Traceback (most recent call last):
   File "/usr/lib/python2.7/dist-packages/twisted/web/wsgi.py", line 315, in run
     appIterator = self.application(self.environ, self.startResponse)
   File "/usr/lib/python2.7/dist-packages/django/core/handlers/wsgi.py", line 206, in __call__
     response = self.get_response(request)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/views.py", line 181, in get_response
     response = get_response(request)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/views.py", line 170, in get_response
     request, get_resolver(None), sys.exc_info())
 --- <exception caught here> ---
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/views.py", line 155, in get_response
     return django_get_response(request)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/orm.py", line 331, in __exit__
     self.fire()
   File "/usr/lib/python2.7/dist-packages/provisioningserver/utils/twisted.py", line 151, in wrapper
     return func(*args, **kwargs)
   File "/usr/lib/python2.7/dist-packages/maasserver/utils/async.py", line 180, in fire
     self._fire_in_reactor(hook).wait()
   File "/usr/lib/python2.7/dist-packages/crochet/_eventloop.py", line 219, in wait
     result.raiseException()
   File "<string>", line 2, in raiseException

 exceptions.AttributeError: 'NoneType' object has no attribute 'encode'