diff --git a/zproject/backends.py b/zproject/backends.py
index e50362ffb8..08833ae9c4 100644
--- a/zproject/backends.py
+++ b/zproject/backends.py
@@ -115,7 +115,7 @@ class ZulipRemoteUserBackend(RemoteUserBackend):
 
     def authenticate(self, remote_user):
         if not remote_user:
-            return
+            return None
 
         email = remote_user_to_email(remote_user)