diff -r c74c412031bb Lib/email/mime/multipart.py
--- a/Lib/email/mime/multipart.py	Sat Jan 19 11:48:13 2008 +0100
+++ b/Lib/email/mime/multipart.py	Sat Jan 19 12:32:14 2008 +0100
@@ -34,6 +34,12 @@ class MIMEMultipart(MIMEBase):
         keyword arguments (or passed into the _params argument).
         """
         MIMEBase.__init__(self, 'multipart', _subtype, **_params)
+        
+        # Initialise _payload to an empty list as the Message superclass's
+        # implementation of is_multipart assumes that _payload is a list for
+        # multipart messages.
+        self._payload = []
+        
         if _subparts:
             for p in _subparts:
                 self.attach(p)
diff -r c74c412031bb Lib/email/test/test_email.py
--- a/Lib/email/test/test_email.py	Sat Jan 19 11:48:13 2008 +0100
+++ b/Lib/email/test/test_email.py	Sat Jan 19 12:27:03 2008 +0100
@@ -1861,6 +1861,9 @@ message 2
         eq(msg.get_payload(0), text1)
         eq(msg.get_payload(1), text2)
 
+    def test_default_multipart_constructor(self):
+        msg = MIMEMultipart()
+        self.assertTrue(msg.is_multipart())
 
 
 # A general test of parser->model->generator idempotency.  IOW, read a message