Index: Lib/ConfigParser.py
===================================================================
--- Lib/ConfigParser.py	(revision 59894)
+++ Lib/ConfigParser.py	(working copy)
@@ -235,8 +235,12 @@
         """Create a new section in the configuration.
 
         Raise DuplicateSectionError if a section by the specified name
-        already exists.
+        already exists. Raise ValueError if name is DEFAULT or any of it's
+        case-insensitive variants.
         """
+        if section.lower() == "default":
+            raise ValueError, 'Invalid section name: %s' % section
+        
         if section in self._sections:
             raise DuplicateSectionError(section)
         self._sections[section] = self._dict()
Index: Lib/test/test_cfgparser.py
===================================================================
--- Lib/test/test_cfgparser.py	(revision 59894)
+++ Lib/test/test_cfgparser.py	(working copy)
@@ -446,6 +446,14 @@
         self.assertRaises(TypeError, cf.set, "sect", "option2", 1.0)
         self.assertRaises(TypeError, cf.set, "sect", "option2", object())
 
+    def test_add_section_default_1(self):
+        cf = self.newconfig()
+        self.assertRaises(ValueError, cf.add_section, "default")
+
+    def test_add_section_default_2(self):
+        cf = self.newconfig()
+        self.assertRaises(ValueError, cf.add_section, "DEFAULT")
+
 class SortedTestCase(RawConfigParserTestCase):
     def newconfig(self, defaults=None):
         self.cf = self.config_class(defaults=defaults, dict_type=SortedDict)
Index: Doc/library/configparser.rst
===================================================================
--- Doc/library/configparser.rst	(revision 59894)
+++ Doc/library/configparser.rst	(working copy)
@@ -187,9 +187,10 @@
 .. method:: RawConfigParser.add_section(section)
 
    Add a section named *section* to the instance.  If a section by the given name
-   already exists, :exc:`DuplicateSectionError` is raised.
+   already exists, :exc:`DuplicateSectionError` is raised. If the name
+   ``DEFAULT`` (or any of it's case-insensitive variants) is passed,
+   :exc:`ValueError` is raised.
 
-
 .. method:: RawConfigParser.has_section(section)
 
    Indicates whether the named section is present in the configuration. The