diff --git 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:

diff --git a/tests/messages/test_mofile.py b/tests/messages/test_mofile.py
@@ -11,8 +11,11 @@
 # history and logs, available at https://github.com/python-babel/babel/commits/master/.
 
 import os
+import struct
 from io import BytesIO
 
+import pytest
+
 from babel.messages import Catalog, mofile
 from babel.support import Translations
 
@@ -104,3 +107,54 @@ def test_read_mo_decodes_message_context() -> None:
     assert message.context == 'ほげ'
     assert message.string == 'åäöÅÄÖ'
     assert not catalog.get('x')  # Not without the context
+
+
+@pytest.mark.parametrize('charset', ['utf-8', 'iso-8859-1'])
+@pytest.mark.parametrize('use_fuzzy', [False, True])
+def test_write_mo_string_tables(charset, use_fuzzy):
+    catalog = Catalog(locale='fr', charset=charset)
+    catalog.add('Coffee', 'Café')
+    catalog.add(('One file', 'Many files'), ('Un fichier', 'Des fichiers'))
+    catalog.add('Open', 'Ouvrir', context='menu')
+    catalog.add('Draft', 'Brouillon', flags=['fuzzy'])
+    catalog.add('Untranslated', '')
+    output = BytesIO()
+
+    mofile.write_mo(output, catalog, use_fuzzy=use_fuzzy)
+
+    data = output.getvalue()
+    _, _, count, key_table, value_table, _, _ = struct.unpack_from('Iiiiiii', data)
+    entries = {}
+    for index in range(count):
+        key_size, key_offset = struct.unpack_from('ii', data, key_table + 8 * index)
+        value_size, value_offset = struct.unpack_from('ii', data, value_table + 8 * index)
+        assert data[key_offset + key_size] == 0
+        assert data[value_offset + value_size] == 0
+        key = data[key_offset:key_offset + key_size].decode(charset)
+        value = data[value_offset:value_offset + value_size].decode(charset)
+        entries[key] = value
+    del entries['']
+    expected = {
+        'Coffee': 'Café',
+        'One file\x00Many files': 'Un fichier\x00Des fichiers',
+        'menu\x04Open': 'Ouvrir',
+    }
+    if use_fuzzy:
+        expected['Draft'] = 'Brouillon'
+    assert entries == expected
+
+
+def test_write_mo_writes_bytes_once():
+    catalog = Catalog()
+    catalog.add('Message', 'Translation')
+    writes = []
+
+    class Output:
+        def write(self, data):
+            assert type(data) is bytes
+            writes.append(data)
+
+    mofile.write_mo(Output(), catalog)
+
+    assert len(writes) == 1
+    assert mofile.read_mo(BytesIO(writes[0]))['Message'].string == 'Translation'