diff --git a/os_service_types/service_types.py b/os_service_types/service_types.py
index 125403e..555425a 100644
--- a/os_service_types/service_types.py
+++ b/os_service_types/service_types.py
@@ -24,7 +24,8 @@ SERVICE_TYPES_URL = "https://service-types.openstack.org/service-types.json"
 
 
 def _normalize_type(service_type):
-    return service_type.replace('_', '-')
+    if service_type:
+        return service_type.replace('_', '-')
 
 
 class ServiceTypes(object):
diff --git a/os_service_types/tests/test_misc.py b/os_service_types/tests/test_misc.py
new file mode 100644
index 0000000..ac42560
--- /dev/null
+++ b/os_service_types/tests/test_misc.py
@@ -0,0 +1,32 @@
+# -*- coding: utf-8 -*-
+
+# Licensed under the Apache License, Version 2.0 (the "License"); you may
+# not use this file except in compliance with the License. You may obtain
+# a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations
+# under the License.
+
+"""
+test_misc
+---------
+
+Miscellaneous tests
+"""
+
+from os_service_types import service_types
+from os_service_types.tests import base
+
+
+class TestMisc(base.TestCase):
+
+    def test_normalize(self):
+        self.assertEqual('foo-bar', service_types._normalize_type('foo_bar'))
+
+    def test_normalize_none(self):
+        self.assertIsNone(service_types._normalize_type(None))