diff --git a/src/zope/i18nmessageid/message.py b/src/zope/i18nmessageid/message.py
index 8bd364f..bd06e8d 100644
--- a/src/zope/i18nmessageid/message.py
+++ b/src/zope/i18nmessageid/message.py
@@ -76,7 +76,7 @@ def __setattr__(self, key, value):
         It cannot be changed once the message id is created.
         """
         if getattr(self, '_readonly', False):
-            raise TypeError('readonly attribute')
+            raise AttributeError('readonly attribute')
         else:
             return str.__setattr__(self, key, value)