diff --git a/IPython/utils/traitlets.py b/IPython/utils/traitlets.py
index eae3f0d..0915520 100644
--- a/IPython/utils/traitlets.py
+++ b/IPython/utils/traitlets.py
@@ -302,8 +302,8 @@ class TraitType(object):
     def __set__(self, obj, value):
         new_value = self._validate(obj, value)
         old_value = self.__get__(obj)
+        obj._trait_values[self.name] = new_value
         if old_value != new_value:
-            obj._trait_values[self.name] = new_value
             obj._notify_trait(self.name, old_value, new_value)
 
     def _validate(self, obj, value):