diff --git a/requests/structures.py b/requests/structures.py
index 5563d1f8..3e5f2faa 100644
--- a/requests/structures.py
+++ b/requests/structures.py
@@ -35,7 +35,7 @@ class CaseInsensitiveDict(collections.MutableMapping):
     of how the header name was originally stored.
 
     If the constructor, ``.update``, or equality comparison
-    operations are given keys that have equal ``.lower()`` s, the
+    operations are given keys that have equal ``.lower()``s, the
     behavior is undefined.
 
     """