diff --git a/oslo_serialization/jsonutils.py b/oslo_serialization/jsonutils.py
index 7df6496..90a5acf 100644
--- a/oslo_serialization/jsonutils.py
+++ b/oslo_serialization/jsonutils.py
@@ -167,7 +167,7 @@ def to_primitive(value, convert_instances=False, convert_datetime=True,
         # __iter__ defined but it isn't callable as list().
         return six.text_type(value)
 
-    return value
+    raise ValueError("Cannot convert %r to primitive" % (value,))
 
 
 JSONEncoder = json.JSONEncoder
diff --git a/oslo_serialization/tests/test_jsonutils.py b/oslo_serialization/tests/test_jsonutils.py
index f649db3..b030b8f 100644
--- a/oslo_serialization/tests/test_jsonutils.py
+++ b/oslo_serialization/tests/test_jsonutils.py
@@ -254,7 +254,7 @@ class ToPrimitiveTestCase(test_base.BaseTestCase):
         self.assertEqual(dict(b=1),
                          jsonutils.to_primitive(x, convert_instances=True))
 
-        self.assertEqual(x, jsonutils.to_primitive(x))
+        self.assertRaises(ValueError, jsonutils.to_primitive, x)
 
     def test_typeerror(self):
         x = bytearray  # Class, not instance