diff --git a/tiramisu/option.py b/tiramisu/option.py
index 284f830..fed7c5d 100644
--- a/tiramisu/option.py
+++ b/tiramisu/option.py
@@ -185,22 +185,26 @@ class Option(HiddenBaseType, DisabledBaseType):
         "who is **not necessarily** a owner because it cannot be a list"
         name = self._name
 
-        if config.is_mandatory() and child.is_mandatory() and \
-                ((self.is_multi() and value == []) or
-                (not self.is_multi() and value is None)):
-            raise MandatoryError('cannot override value to %s for '
-              'option %s' % (value, name))
-        if name not in config._cfgimpl_values:
-            raise AttributeError('unknown option %s' % (name))
-        if config.is_frozen() and (child.has_callback() or child.isfrozen()):
-            raise ConflictConfigError('cannot override value to %s for '
-              'option %s' % (value, name))
         # we want the possibility to reset everything
         if who == "default" and value is None:
             self.default = None
             return
         if not self.validate(value):
             raise ConfigError('invalid value %s for option %s' % (value, name))
+        if self.is_mandatory():
+            # value shall not be '' for a mandatory option
+            # so '' is considered as being None
+            if value == '':
+                value = None
+            if config.is_mandatory() and ((self.is_multi() and value == []) or 
+                (not self.is_multi() and value is None)):
+                raise MandatoryError('cannot override value to %s for '
+              'option %s' % (value, name))
+        if name not in config._cfgimpl_values:
+            raise AttributeError('unknown option %s' % (name))
+        if config.is_frozen() and (self.has_callback() or self.isfrozen()):
+            raise ConflictConfigError('cannot override value to %s for '
+              'option %s' % (value, name))
         if who == "default":
             # changes the default value (and therefore resets the previous value)
             if self._validate(value):
@@ -240,7 +244,7 @@ class Option(HiddenBaseType, DisabledBaseType):
     def unfreeze(self):
         self._frozen = False
 
-    def isfrozen(self):
+    def is_frozen(self):
         return self._frozen
     # ____________________________________________________________
     def is_multi(self):
@@ -527,6 +531,7 @@ def apply_requires(opt, config):
                         if action not in available_actions:
                             raise RequiresError("malformed requirements"
                                            " for option: {0}".format(opt._name))
+                        # FIXME generic programming opt.property_launch(action, False)
                         getattr(opt, action)() #.hide() or show() or...
                         matches = True
                 else: # option doesn't exist ! should not happen...