diff --git a/requests/structures.py b/requests/structures.py
index 2addb938..a710e9bb 100644
--- a/requests/structures.py
+++ b/requests/structures.py
@@ -109,7 +109,7 @@ class CaseInsensitiveDict(collections.MutableMapping):
         return '%s(%r)' % (self.__class__.__name__, dict(self.items()))
 
     def __str__(self):
-        return '%s' % (dict(self.items())
+        return '%s' % (dict(self.items()))
 
 
 class LookupDict(dict):