diff --git a/mailman_sync.py b/mailman_sync.py
index 6ed93f3..59c272e 100755
--- a/mailman_sync.py
+++ b/mailman_sync.py
@@ -12,7 +12,7 @@ import requests
 
 
 def sync_members(mailman_bin: Path, mailing_list: str, members: list[str]):
-    members_file = "\n".join(members)
+    members_data = "\n".join(members).encode('ascii')
     output = subprocess.run(
         [
             mailman_bin / "sync_members",
@@ -24,7 +24,7 @@ def sync_members(mailman_bin: Path, mailing_list: str, members: list[str]):
             "-",
             mailing_list,
         ],
-        input=members_file,
+        input=members_data,
         capture_output=True,
         check=True,
     )