diff --git a/tiramisu/setting.py b/tiramisu/setting.py
index 5209b72..dd00686 100644
--- a/tiramisu/setting.py
+++ b/tiramisu/setting.py
@@ -130,16 +130,16 @@ FORBIDDEN_SET_PERMISSIVES = frozenset(['force_default_on_freeze',
                                        'force_metaconfig_on_freeze',
                                        'force_store_value',
                                        ])
-ALLOWED_LEADER_PROPERTIES = frozenset(['empty',
-                                       'notempty',
-                                       'notunique',
-                                       'unique',
-                                       'force_store_value',
-                                       'mandatory',
-                                       'force_default_on_freeze',
-                                       'force_metaconfig_on_freeze',
-                                       'frozen',
-                                       ])
+ALLOWED_LEADER_PROPERTIES = {'empty',
+                             'notempty',
+                             'notunique',
+                             'unique',
+                             'force_store_value',
+                             'mandatory',
+                             'force_default_on_freeze',
+                             'force_metaconfig_on_freeze',
+                             'frozen',
+                             }
 
 static_set = frozenset()