diff --git a/neutron/callbacks/exceptions.py b/neutron/callbacks/exceptions.py
index f4cded227ed..96dbef52abb 100644
--- a/neutron/callbacks/exceptions.py
+++ b/neutron/callbacks/exceptions.py
@@ -23,7 +23,10 @@ class CallbackFailure(Exception):
         self.errors = errors
 
     def __str__(self):
-        return ','.join(str(error) for error in self.errors)
+        if isinstance(self.errors, list):
+            return ','.join(str(error) for error in self.errors)
+        else:
+            return str(self.errors)
 
 
 class NotificationError(object):
diff --git a/tox.ini b/tox.ini
index 2d3b5740829..f5094b0cc1f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -160,6 +160,7 @@ commands = python -m testtools.run \
     neutron.tests.unit.db.test_agents_db \
     neutron.tests.unit.db.quota.test_driver \
     neutron.tests.unit.db.test_dvr_mac_db \
+    neutron.tests.unit.db.test_securitygroups_db \
     neutron.tests.unit.debug.test_commands \
     neutron.tests.unit.tests.test_post_mortem_debug \
     neutron.tests.unit.tests.test_base \