diff --git a/node_external_ip_controller_async.py b/node_external_ip_controller_async.py
index 66c4148..0894793 100644
--- a/node_external_ip_controller_async.py
+++ b/node_external_ip_controller_async.py
@@ -39,7 +39,8 @@ async def update_service_annotation(v1, service, external_ipset):
         logger.debug(f"Zlan Gateway IP: {zlan_gateway_ip}")
 
         if is_valid_ip(zlan_gateway_ip):
-            external_ipset = set(external_ipset).add(zlan_gateway_ip)
+            external_ipset = set(external_ipset)
+            external_ipset.add(zlan_gateway_ip)
         else:
             logger.debug(
                 f"Invalid Zlan Gateway IP: {zlan_gateway_ip}, excluding from target annotation"