diff --git a/src/requests/structures.py b/src/requests/structures.py
index 188e13e482..993cf2459a 100644
--- a/src/requests/structures.py
+++ b/src/requests/structures.py
@@ -55,7 +55,7 @@ def __delitem__(self, key):
         del self._store[key.lower()]
 
     def __iter__(self):
-        return (casedkey for casedkey, mappedvalue in self._store.values())
+        return (casedkey for casedkey in self._store)
 
     def __len__(self):
         return len(self._store)