diff --git a/synapse/config/server.py b/synapse/config/server.py
index 493d2bd513..b5c2098ecb 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -541,8 +541,8 @@ 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: