diff --git a/synapse/config/server.py b/synapse/config/server.py
index 23ddd0ab0..493d2bd51 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -541,7 +541,9 @@ class ServerConfig(Config):
             raise ConfigError("'users_new_default_push_rules' must be a list")
 
         # Turn the list into a set to improve lookup speed.
-        self.users_new_default_push_rules = set(self.users_new_default_push_rules)
+        self.users_new_default_push_rules = (
+            set(self.users_new_default_push_rules)
+        )  # type: set
 
     def has_tls_listener(self) -> bool:
         return any(listener.tls for listener in self.listeners)