Index: Lib/ConfigParser.py
===================================================================
--- Lib/ConfigParser.py	(revision 83912)
+++ Lib/ConfigParser.py	(working copy)
@@ -699,13 +699,13 @@
         if self._optcre is self.OPTCRE or value:
             if not isinstance(value, basestring):
                 raise TypeError("option values must be strings")
-        # check for bad percent signs:
-        # first, replace all "good" interpolations
-        tmp_value = value.replace('%%', '')
-        tmp_value = self._interpvar_re.sub('', tmp_value)
-        # then, check if there's a lone percent sign left
-        percent_index = tmp_value.find('%')
-        if percent_index != -1:
-            raise ValueError("invalid interpolation syntax in %r at "
-                             "position %d" % (value, percent_index))
+        if value is not None:
+            # check for bad percent signs:
+            # first, replace all "good" interpolations
+            tmp_value = value.replace('%%', '')
+            tmp_value = self._interpvar_re.sub('', tmp_value)
+            # then, check if there's a lone percent sign left
+            if '%' in tmp_value:
+                raise ValueError("invalid interpolation syntax in %r at "
+                                "position %d" % (value, tmp_value.find('%')))
         ConfigParser.set(self, section, option, value)
Index: Lib/test/test_cfgparser.py
===================================================================
--- Lib/test/test_cfgparser.py	(revision 83912)
+++ Lib/test/test_cfgparser.py	(working copy)
@@ -357,6 +357,7 @@
 
 class ConfigParserTestCase(TestCaseBase):
     config_class = ConfigParser.ConfigParser
+    allow_no_value = True
 
     def test_interpolation(self):
         rawval = {
@@ -397,6 +398,7 @@
         cf.set('non-string', 'dict', {'pi': 3.14159, '%(': 1,
                                       '%(list)': '%(list)'})
         cf.set('non-string', 'string_with_interpolation', '%(list)s')
+        cf.set('non-string', 'no-value')
         self.assertEqual(cf.get('non-string', 'int', raw=True), 1)
         self.assertRaises(TypeError, cf.get, 'non-string', 'int')
         self.assertEqual(cf.get('non-string', 'list', raw=True),
@@ -409,6 +411,7 @@
                                 raw=True), '%(list)s')
         self.assertRaises(ValueError, cf.get, 'non-string',
                           'string_with_interpolation', raw=False)
+        self.assertEqual(cf.get('non-string', 'no-value'), None)
 
 class MultilineValuesTestCase(TestCaseBase):
     config_class = ConfigParser.ConfigParser