diff --git a/option.py b/option.py
index e305d4a..bbc471d 100644
--- a/option.py
+++ b/option.py
@@ -81,7 +81,7 @@ class Option(HiddenBaseType, DisabledBaseType, ModeBaseType):
         if who == "default" and value is None:
             self.default = None
             return
-        if not self.validate(value):
+        if value != None and not self.validate(value):
             raise ConfigError('invalid value %s for option %s' % (value, name))
         if who == "default":
             # changes the default value (and therefore resets the previous value)
diff --git a/test/test_config.py b/test/test_config.py
index a18cf60..577488a 100644
--- a/test/test_config.py
+++ b/test/test_config.py
@@ -33,6 +33,14 @@ def test_base_config():
     assert cfg.dummy == False
     dm = cfg.unwrap_from_path('dummy')
     assert dm._name == 'dummy'
+
+def test_toto():
+    descr = make_description()
+    cfg = Config(descr)
+    assert cfg.gc.dummy == False
+    cfg.gc.dummy = True
+    assert cfg.gc.dummy == True    
+    cfg.gc.dummy = None
     
 def test_base_config_and_groups():
     descr = make_description()