diff --git a/requests/sessions.py b/requests/sessions.py
index 697ce2ef..b29f13e5 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -60,8 +60,9 @@ def merge_kwargs(local_kwarg, default_kwarg):
         return new_key
 
     kwargs = default_kwarg.copy()
+    original_keys = kwargs.keys()
     for key, value in local_kwarg.items():
-        kwargs[get_original_key(kwargs.keys(), key)] = value
+        kwargs[get_original_key(original_keys, key)] = value
 
     # Remove keys that are set to None.
     for (k, v) in local_kwarg.items():