diff --git a/tiramisu/config.py b/tiramisu/config.py
index 58ff788..c97fb60 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -188,16 +188,18 @@ class Config(object):
     def _validate(self, name, opt_or_descr):
         apply_requires(opt_or_descr, self) 
         if not type(opt_or_descr) == OptionDescription:
-            # hidden or disabled options
-            # XXX let's have a group with a hidden property 
-            # and an option with a disabled property, 
-            # then it matches anyway... is it important to fix this ?
             if self._cfgimpl_toplevel._cfgimpl_has_properties() and \
-                (opt_or_descr.has_properties() or \
-                self._cfgimpl_descr.has_properties()):
-                raise PropertiesOptionError("trying to access to the option: {0} "
-                    "with properties: {1}".format(name, 
-                    str(opt_or_descr.properties)))
+                    opt_or_descr.has_properties():
+                raise PropertiesOptionError("trying to access"
+                        " to an option named: {0}".format(name), 
+                        opt_or_descr.properties))
+            if self._cfgimpl_toplevel._cfgimpl_has_properties() and \
+                    self._cfgimpl_descr.has_properties()):
+                raise PropertiesOptionError("trying to access"
+                        " to an option's group named: {0}"
+                        " for option named: {1}".format(
+                            self._cfgimpl_descr._name, name), 
+                        self._cfgimpl_descr.properties)
 
     def __getattr__(self, name):
         # attribute access by passing a path, 
@@ -223,7 +225,8 @@ class Config(object):
             # options with callbacks (fill or auto) 
             if opt_or_descr.has_callback():
                 value = self._cfgimpl_values[name]
-                if value != None:
+                if (not opt_or_descr.is_frozen() or \
+                        not opt_or_descr.is_forced_on_freeze()) and value != None:
                     if opt_or_descr.is_multi():
                         if None not in value:
                             return value
diff --git a/tiramisu/error.py b/tiramisu/error.py
index 061099b..07cc0db 100644
--- a/tiramisu/error.py
+++ b/tiramisu/error.py
@@ -7,7 +7,10 @@ class ConfigError(Exception):
 class ConflictConfigError(ConfigError):
     pass
 class PropertiesOptionError(AttributeError):
-    pass
+    def __init__(self, msg, proptype=None):
+        self.proptype = proptype
+        super(PropertiesOptionError, self).__init__(msg)
+
 class NotFoundError(Exception):
     pass
 class MethodCallError(Exception):