Index: Doc/library/nntplib.rst
===================================================================
--- Doc/library/nntplib.rst	(révision 86127)
+++ Doc/library/nntplib.rst	(copie de travail)
@@ -252,6 +252,8 @@
      (including headers and body)
    * the ``:lines`` metadata: the number of lines in the article body
 
+   The value of each item is either a string, or :const:`None` if not present.
+
    It is advisable to use the :func:`decode_header` function on header
    values when they may contain non-ASCII characters::
 
Index: Lib/nntplib.py
===================================================================
--- Lib/nntplib.py	(révision 86129)
+++ Lib/nntplib.py	(copie de travail)
@@ -205,11 +205,12 @@
             is_metadata = field_name.startswith(':')
             if i >= n_defaults and not is_metadata:
                 # Non-default header names are included in full in the response
-                h = field_name + ":"
-                if token[:len(h)].lower() != h:
+                # (unless the field is totally empty)
+                h = field_name + ": "
+                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[len(h):] if token else 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 +1030,29 @@
             ':lines': '17',
             'xref': 'news.example.com misc.test:3000363',
         })
+        # Second example; here the "Xref" field is totally absent (including
+        # the header name) and comes 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(fields['xref'], None)
+        # Third example; the "Xref" is an empty string, while "references"
+        # is a single space.
+        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 \t1234\t'
+            '17\tXref: \t',
+        ]
+        overview = nntplib._parse_overview(lines, fmt)
+        (art_num, fields), = overview
+        self.assertEqual(fields['references'], ' ')
+        self.assertEqual(fields['xref'], '')
 
     def test_parse_datetime(self):
         def gives(a, b, *c):