Index: Lib/email/test/data/msg_46.txt
===================================================================
--- Lib/email/test/data/msg_46.txt	(revision 0)
+++ Lib/email/test/data/msg_46.txt	(revision 0)
@@ -0,0 +1,23 @@
+Return-Path: <sender@example.net>
+Delivery-Date: Mon, 08 Feb 2010 14:05:16 +0100
+Received: from example.org (example.org [64.5.53.58])
+    by example.net (node=mxbap2) with ESMTP (Nemesis)
+    id UNIQUE for someone@example.com; Mon, 08 Feb 2010 14:05:16 +0100
+Date: Mon, 01 Feb 2010 12:21:16 +0100
+From: "Sender" <sender@example.net>
+To: <someone@example.com>
+Subject: GroupwiseForwardingTest
+Mime-Version: 1.0
+Content-Type: message/rfc822
+
+Return-path: <sender@example.net>
+Message-ID: <4B66B890.4070408@teconcept.de>
+Date: Mon, 01 Feb 2010 12:18:40 +0100
+From: "Dr. Sender" <sender@example.net>
+MIME-Version: 1.0
+To: "Recipient" <recipient@example.com>
+Subject: GroupwiseForwardingTest
+Content-Type: text/plain; charset=ISO-8859-15
+Content-Transfer-Encoding: 7bit
+
+Testing email forwarding with Groupwise 1.2.2010
Index: Lib/email/test/test_email.py
===================================================================
--- Lib/email/test/test_email.py	(revision 78215)
+++ Lib/email/test/test_email.py	(working copy)
@@ -179,6 +179,18 @@
         self.assertRaises(Errors.HeaderParseError,
                           msg.set_boundary, 'BOUNDARY')
 
+    def test_message_rfc822_only(self):
+        # Issue 7970: message/rfc822 not in multipart parsed by
+        # HeaderParser caused an exception when flattened.
+        fp = openfile(findfile('msg_46.txt'))
+        msgdata = fp.read()
+        parser = email.Parser.HeaderParser()
+        msg = parser.parsestr(msgdata)
+        out = StringIO()
+        gen = email.Generator.Generator(out, True, 0)
+        gen.flatten(msg, False)
+        self.assertEqual(out.getvalue(), msgdata)
+
     def test_get_decoded_payload(self):
         eq = self.assertEqual
         msg = self._msgobj('msg_10.txt')
Index: Lib/email/generator.py
===================================================================
--- Lib/email/generator.py	(revision 78215)
+++ Lib/email/generator.py	(working copy)
@@ -276,8 +276,16 @@
         # of length 1.  The zeroth element of the list should be the Message
         # object for the subpart.  Extract that object, stringify it, and
         # write it out.
-        g.flatten(msg.get_payload(0), unixfrom=False)
-        self._fp.write(s.getvalue())
+        # Except, it turns out, when it's a string instead, which happens when
+        # and only when HeaderParser is used on a message of mime type
+        # message/rfc822.  Such messages are generated by, for example,
+        # Groupwise when forwarding unadorned messages.  (Issue 7970.)  So
+        # in that case we just emit the string body.
+        payload = msg.get_payload()
+        if isinstance(payload, list):
+            g.flatten(msg.get_payload(0), unixfrom=False)
+            payload = s.getvalue()
+        self._fp.write(payload)