should not be possible to delete used flavors

At the moment it's possible (tested with Essex) to delete an flavor that's used by an active instance. After deleting the flavor I have problems with several API calls, for example I can't list instances because I receive the following error:

2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi Traceback (most recent call last):
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/wsgi.py", line 851, in _process_stack
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi action_result = self.dispatch(meth, request, action_args)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/wsgi.py", line 926, in dispatch
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi return method(req=request, **action_args)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/servers.py", line 382, in detail
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi servers = self._get_servers(req, is_detail=True)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/servers.py", line 465, in _get_servers
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi return self._view_builder.detail(req, limited_list)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/views/servers.py", line 123, in detail
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi return self._list_view(self.show, request, instances)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/views/servers.py", line 127, in _list_view
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi server_list = [func(request, server)["server"] for server in servers]
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/views/servers.py", line 61, in wrapped
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi return func(self, request, instance)
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/views/servers.py", line 97, in show
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi "flavor": self._get_flavor(request, instance),
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi File "/usr/lib64/python2.6/site-packages/nova/api/openstack/compute/views/servers.py", line 172, in _get_flavor
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi flavor_id = instance["instance_type"]["flavorid"]
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi TypeError: 'NoneType' object is unsubscriptable
2012-05-16 18:54:26 TRACE nova.api.openstack.wsgi