diff --git a/option.py b/option.py
index 7e7fc16..d32101d 100644
--- a/option.py
+++ b/option.py
@@ -46,15 +46,22 @@ class Multi(list):
             return self.child.default_multi
             
     def __setitem__(self, key, value):
+        if value is None:
+            owner = 'default'
+        else:
+            owner = default_owner
+        self.child.setowner(self.config, owner)
         if value != None and not self.child._validate(value):
             raise ConfigError("invalid value {0} "
                 "for option {1}".format(str(value), self.child._name))
         # FIXME : and if value is None ???
         return super(Multi, self).__setitem__(key, value)
 
-    def append(self, value, owner=default_owner):
-        if owner is None:
+    def append(self, value):
+        if value is None:
             owner = 'default'
+        else:
+            owner = default_owner
         self.child.setowner(self.config, owner)
         # changer dans la config la valeur par défaut et le owner
         if value != None and not self.child._validate(value):