diff --git a/mailman_sync.py b/mailman_sync.py
index 3373bb4..aa0ed42 100755
--- a/mailman_sync.py
+++ b/mailman_sync.py
@@ -68,7 +68,7 @@ def diff_roster(
     expected_members_dict = dict(
         list(reversed(email.utils.parseaddr(member))) for member in expected_members
     )
-    existing_members_dict = {member.address: member for member in existing_members}
+    existing_members_dict = {str(member.address): member for member in existing_members}
     members_to_add = {
         k: expected_members_dict[k]
         for k in set(expected_members_dict) - set(existing_members_dict)