diff --git a/src/dbus_fast/_private/marshaller.py b/src/dbus_fast/_private/marshaller.py
index 8ec71be..abd886d 100644
--- a/src/dbus_fast/_private/marshaller.py
+++ b/src/dbus_fast/_private/marshaller.py
@@ -31,7 +31,8 @@ class Marshaller:
         offset = n - len(self._buf) % n
         if offset == 0 or offset == n:
             return 0
-        self._buf.extend(bytes(offset))
+        for _ in range(offset):
+            self._buf.append(0)
         return offset
 
     def write_boolean(self, boolean: bool, type_: SignatureType) -> int: