diff --git a/proteus/config.py b/proteus/config.py
index 30593369c8..5202f3b7d2 100644
--- a/proteus/config.py
+++ b/proteus/config.py
@@ -51,7 +51,7 @@ class Config(object):
     def __eq__(self, other):
         if isinstance(other, Config):
             return repr(self) == repr(other)
-        return NotImplemented
+        return NotImplementedError
 
 
 class _TrytondMethod(object):
diff --git a/proteus/tests/test_config.py b/proteus/tests/test_config.py
index 8c568b49dd..35433a38c1 100644
--- a/proteus/tests/test_config.py
+++ b/proteus/tests/test_config.py
@@ -26,4 +26,4 @@ class TestConfig(TestCase):
         config2 = proteus.config.get_config()
         self.assertEqual(config1, config2)
 
-        self.assertEqual(config1.__eq__(None), NotImplemented)
+        self.assertRaises(NotImplementedError, config1.__eq__, None)