diff --git a/charms_openstack/charm/defaults.py b/charms_openstack/charm/defaults.py
index 81f870e..3f9a676 100644
--- a/charms_openstack/charm/defaults.py
+++ b/charms_openstack/charm/defaults.py
@@ -103,6 +103,8 @@ def make_default_select_release_handler():
                 # First make an attempt of determining release from a charm
                 # instance defined package codename dictionary.
                 singleton = get_charm_instance()
+                if singleton.release_pkg is None:
+                    raise RuntimeError("release_pkg is not set")
                 release_version = singleton.get_os_codename_package(
                     singleton.release_pkg, singleton.package_codenames,
                     apt_cache_sufficient=(not singleton.source_config_key))
diff --git a/unit_tests/charms_openstack/charm/test_defaults.py b/unit_tests/charms_openstack/charm/test_defaults.py
index bd34080..c2efa5e 100644
--- a/unit_tests/charms_openstack/charm/test_defaults.py
+++ b/unit_tests/charms_openstack/charm/test_defaults.py
@@ -119,6 +119,12 @@ class TestDefaults(BaseOpenStackCharmTest):
         self.assertEqual(release, 'one')
         kv.set.assert_not_called()
         kv.get.assert_called_once_with(chm.OPENSTACK_RELEASE_KEY, None)
+        # No release_pkg set, ensure a RuntimeError raised
+        kv.get.return_value = None
+        singleton.release_pkg = None
+        with self.assertRaises(RuntimeError):
+            h.map['function']()
+        singleton.release_pkg = "my-pkg"
         # No release set, ensure it calls os_release
         kv.reset_mock()
         kv.get.return_value = None