diff --git a/httpbin/structures.py b/httpbin/structures.py
index c57ff73..44da70f 100644
--- a/httpbin/structures.py
+++ b/httpbin/structures.py
@@ -16,7 +16,7 @@ class CaseInsensitiveDict(dict):
     """
 
     def _lower_keys(self):
-        return [str.lower(k) for k in  self.keys()]
+        return [k.lower() for k in self.keys()]
 
     def __contains__(self, key):
         return key.lower() in self._lower_keys()