diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 3b49a74..a66b555 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -380,7 +380,7 @@
 
    .. method:: defaultdict.__missing__(key)
 
-      If the :attr:`default_factory` attribute is ``None``, this raises an
+      If the :attr:`default_factory` attribute is ``None``, this raises a
       :exc:`KeyError` exception with the *key* as argument.
 
       If :attr:`default_factory` is not ``None``, it is called without arguments