diff --git a/tiramisu/config.py b/tiramisu/config.py
index fd19b82..aa752ab 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -175,9 +175,7 @@ class Config(object):
             raise MethodCallError("this method root_mandatory machin() shall not be"
                                            "used with non-root Confit() object") 
         rootconfig = self._cfgimpl_get_toplevel()
-        if 'mandatory' in rootconfig._cfgimpl_properties:
-            rootconfig._cfgimpl_properties.remove('mandatory')
-
+        rootconfig._cfgimpl_mandatory = False
 
     # ____________________________________________________________
     def __setattr__(self, name, value):