Index: Lib/nntplib.py
===================================================================
--- Lib/nntplib.py	(révision 86129)
+++ Lib/nntplib.py	(copie de travail)
@@ -205,11 +205,13 @@
             is_metadata = field_name.startswith(':')
             if i >= n_defaults and not is_metadata:
                 # Non-default header names are included in full in the response
+                # (unless the field is totally empty)
                 h = field_name + ":"
-                if token[:len(h)].lower() != h:
+                if token and token[:len(h)].lower() != h:
                     raise NNTPDataError("OVER/XOVER response doesn't include "
                                         "names of additional headers")
                 token = token[len(h):].lstrip(" ")
+                token = token or None
             fields[fmt[i]] = token
         overview.append((article_number, fields))
     return overview
Index: Lib/test/test_nntplib.py
===================================================================
--- Lib/test/test_nntplib.py	(révision 86129)
+++ Lib/test/test_nntplib.py	(copie de travail)
@@ -457,7 +457,7 @@
                     "\tThu, 22 Jul 2010 09:14:14 -0400"
                     "\t<A29863FA-F388-40C3-AA25-0FD06B09B5BF@gmail.com>"
                     "\t\t6683\t16"
-                    "\tXref: news.gmane.org gmane.comp.python.authors:58"
+                    "\t"
                     "\n"
                 # An UTF-8 overview line from fr.comp.lang.python
                 "59\tRe: Message d'erreur incompréhensible (par moi)"
@@ -824,6 +824,8 @@
             ":lines": "16",
             "xref": "news.gmane.org gmane.comp.python.authors:57"
             })
+        art_num, over = overviews[1]
+        self.assertEqual(over["xref"], None)
         art_num, over = overviews[2]
         self.assertEqual(over["subject"],
                          "Re: Message d'erreur incompréhensible (par moi)")
@@ -1028,6 +1030,27 @@
             ':lines': '17',
             'xref': 'news.example.com misc.test:3000363',
         })
+        # Second example; here the "Xref" field is totally absent (including
+        # the header name) and will come out as None
+        lines = [
+            '3000234\tI am just a test article\t"Demo User" '
+            '<nobody@example.com>\t6 Oct 1998 04:38:40 -0500\t'
+            '<45223423@example.com>\t<45454@example.net>\t1234\t'
+            '17\t\t',
+        ]
+        overview = nntplib._parse_overview(lines, fmt)
+        (art_num, fields), = overview
+        self.assertEqual(art_num, 3000234)
+        self.assertEqual(fields, {
+            'subject': 'I am just a test article',
+            'from': '"Demo User" <nobody@example.com>',
+            'date': '6 Oct 1998 04:38:40 -0500',
+            'message-id': '<45223423@example.com>',
+            'references': '<45454@example.net>',
+            ':bytes': '1234',
+            ':lines': '17',
+            'xref': None,
+        })
 
     def test_parse_datetime(self):
         def gives(a, b, *c):