diff --git a/silk/model_factory.py b/silk/model_factory.py
index 6a1fd180..a802beb4 100644
--- a/silk/model_factory.py
+++ b/silk/model_factory.py
@@ -66,20 +66,16 @@ def encoded_headers(self):
         """
         From Django docs (https://docs.djangoproject.com/en/2.0/ref/request-response/#httprequest-objects):
         """
-        sensitive_headers = set(SilkyConfig().SILKY_SENSITIVE_KEYS)
-
+        sensitive_headers = set(map(str.lower, SilkyConfig().SILKY_SENSITIVE_KEYS))
         sensitive_headers.add('authorization')
         if SilkyConfig().SILKY_HIDE_COOKIES:
             sensitive_headers.add('cookie')
 
-        sensitive_headers = set(map(str.lower, sensitive_headers))
-
         headers = {}
         for k, v in self.request.headers.items():
             k = k.lower()
             if k in sensitive_headers:
                 v = RequestModelFactory.CLEANSED_SUBSTITUTE
-
             headers[k] = v
 
         return json.dumps(headers, cls=DefaultEncoder, ensure_ascii=SilkyConfig().SILKY_JSON_ENSURE_ASCII)