Index: Lib/email/test/test_email.py
===================================================================
--- Lib/email/test/test_email.py	(revision 77712)
+++ Lib/email/test/test_email.py	(working copy)
@@ -103,6 +103,11 @@
         eq(msg['content-type'], 'text/x-weird; charset="iso-8859-1"')
         eq(msg['content-transfer-encoding'], 'quinted-puntable')
 
+    def test_set_charset_non_text_maintype(self):
+        msg = MIMEMultipart()
+        msg.attach(MIMEText('foo', 'plain'))
+        self.assertRaises(TypeError, msg.set_charset, 'utf-8')
+
     def test_set_charset_from_string(self):
         eq = self.assertEqual
         msg = Message()
Index: Lib/email/message.py
===================================================================
--- Lib/email/message.py	(revision 77712)
+++ Lib/email/message.py	(working copy)
@@ -238,6 +238,9 @@
         Content-Type, Content-Transfer-Encoding) will be added as needed.
 
         """
+        if self.get_content_maintype() != 'text':
+            raise TypeError(
+                "set_charset() called on a message not of type text/*.")
         if charset is None:
             self.del_param('charset')
             self._charset = None