diff --git a/bundles/postfix/metadata.py b/bundles/postfix/metadata.py
index a44c4b3..e381728 100644
--- a/bundles/postfix/metadata.py
+++ b/bundles/postfix/metadata.py
@@ -73,7 +73,7 @@ def letsencrypt(metadata):
 
     if myhostname and myhostname != metadata.get('hostname'):
         result['domains'] = {
-            myhostname: {},
+            myhostname: set(),
         }
 
     return {