diff --git a/tests/test_transport.py b/tests/test_transport.py
index d045aaf..cbcdecd 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -140,7 +140,7 @@ and I'm all out of gum."""),
         ("", {}, "\n\n\n\n\n\n"),
         ("", {}, u("\u4E2D\u539F\u5343\u519B\u9010\u848B"))))
     def test_string_representation_with_message(self, url, headers, message):
-        for key, value in headers.items():
+        for key, value in list(headers.items()):
             old_key = key
             if isinstance(key, text_type):
                 key = key.encode("utf-8")