diff --git a/requests/structures.py b/requests/structures.py
index f49ccdcf..59d4645e 100644
--- a/requests/structures.py
+++ b/requests/structures.py
@@ -38,6 +38,8 @@ class CaseInsensitiveDict(collections.MutableMapping):
     behavior is undefined.
     """
 
+    __slots__ = ('_store')
+
     def __init__(self, data=None, **kwargs):
         self._store = collections.OrderedDict()
         if data is None:
@@ -96,7 +98,7 @@ class HTTPHeaderDict(CaseInsensitiveDict):
         self.extend({} if data is None else data, **kwargs)
 
 
-    # 
+    #
     # We'll store tuples in the internal dictionary, but present them as a
     # concatenated string when we use item access methods.
     #