diff --git a/acme/acme/messages.py b/acme/acme/messages.py
index c824c43cf..1d907e5fc 100644
--- a/acme/acme/messages.py
+++ b/acme/acme/messages.py
@@ -245,7 +245,7 @@ class Directory(jose.JSONDeSerializable):
         try:
             return self[name.replace('_', '-')]
         except KeyError as error:
-            raise AttributeError(str(error) + ': ' + name)
+            raise AttributeError(str(error))
 
     def __getitem__(self, name):
         try: