diff --git a/babel/messages/mofile.py b/babel/messages/mofile.py
index 2413f7139..164d0b41b 100644
--- a/babel/messages/mofile.py
+++ b/babel/messages/mofile.py
@@ -163,7 +163,8 @@ def write_mo(fileobj: SupportsWrite[bytes], catalog: Catalog, use_fuzzy: bool =
     messages[1:] = [m for m in messages[1:] if m.string and (use_fuzzy or not m.fuzzy)]
     messages.sort()
 
-    ids = strs = b''
+    ids = bytearray()
+    strs = bytearray()
     offsets = []
 
     for message in messages: