diff --git a/notebook/base/handlers.py b/notebook/base/handlers.py
index 6be7a41ff..701115f33 100755
--- a/notebook/base/handlers.py
+++ b/notebook/base/handlers.py
@@ -68,13 +68,14 @@ class AuthenticatedHandler(web.RequestHandler):
         ])
 
     def set_default_headers(self):
-        headers = self.settings.get('headers', {})
+        headers = {}
+        headers.update(self.settings.get('headers', {}))
 
         if "Content-Security-Policy" not in headers:
             headers["Content-Security-Policy"] = self.content_security_policy
-
+        
         # Allow for overriding headers
-        for header_name,value in headers.items() :
+        for header_name, value in headers.items():
             try:
                 self.set_header(header_name, value)
             except Exception as e: