diff -up ./src/mailman/handlers/to_digest.py.digestunicode ./src/mailman/handlers/to_digest.py
--- ./src/mailman/handlers/to_digest.py.digestunicode	2013-04-18 12:59:09.078048003 +0000
+++ ./src/mailman/handlers/to_digest.py	2013-04-18 13:37:44.606048003 +0000
@@ -55,7 +55,7 @@ class ToDigest:
         mailbox_path = os.path.join(mlist.data_path, 'digest.mmdf')
         # Lock the mailbox and append the message.
         with Mailbox(mailbox_path, create=True) as mbox:
-            mbox.add(msg)
+            mbox.add(msg.as_string())
         # Calculate the current size of the mailbox file.  This will not tell
         # us exactly how big the resulting MIME and rfc1153 digest will
         # actually be, but it's the most easily available metric to decide
diff -up ./src/mailman/handlers/tests/test_to_digest.py.digestunicode ./src/mailman/handlers/tests/test_to_digest.py
--- ./src/mailman/handlers/tests/test_to_digest.py.digestunicode	2013-04-18 12:58:57.267047922 +0000
+++ ./src/mailman/handlers/tests/test_to_digest.py	2013-04-18 13:14:55.912047953 +0000
@@ -0,0 +1,63 @@
+# Copyright (C) 2012-2013 by the Free Software Foundation, Inc.
+#
+# This file is part of GNU Mailman.
+#
+# GNU Mailman is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation, either version 3 of the License, or (at your option)
+# any later version.
+#
+# GNU Mailman is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+# more details.
+#
+# You should have received a copy of the GNU General Public License along with
+# GNU Mailman.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Test the to_digest handler."""
+
+from __future__ import absolute_import, print_function, unicode_literals
+
+__metaclass__ = type
+__all__ = [
+    'TestToDigest',
+    ]
+
+
+import unittest
+import os
+
+from mailman.app.lifecycle import create_list
+from mailman.testing.helpers import (
+    get_queue_messages,
+    specialized_message_from_string as mfs)
+from mailman.testing.layers import ConfigLayer
+
+from mailman.handlers.to_digest import ToDigest
+
+
+
+class TestToDigest(unittest.TestCase):
+    """Test the to_digest handler."""
+
+    layer = ConfigLayer
+
+    def setUp(self):
+        self._mlist = create_list('test@example.com')
+        self._msg = mfs("""\
+From: anne@example.com
+To: test@example.com
+Subject: A disposable message
+Message-ID: <ant>
+
+""")
+        self._handler = ToDigest()
+
+    def test_unicode_message(self):
+        self._msg.set_payload(b"non-ascii chars \xc3\xa9 \xc3\xa8 \xc3\xa7")
+        self._msg["X-Test"] = "dummy"
+        self._handler.process(self._mlist, self._msg, {})
+        # Make sure the digest mbox is not empty
+        mailbox_path = os.path.join(self._mlist.data_path, 'digest.mmdf')
+        self.assertNotEqual(os.path.getsize(mailbox_path), 0)