diff -r 3fbfa61634de Lib/unittest/loader.py
--- a/Lib/unittest/loader.py	Thu Jul 19 00:14:35 2012 -0500
+++ b/Lib/unittest/loader.py	Thu Jul 19 19:54:06 2012 -0300
@@ -211,6 +211,7 @@
             raise ImportError('Start directory is not importable: %r' % start_dir)
 
         tests = list(self._find_tests(start_dir, pattern))
+        self._top_level_dir = None
         return self.suiteClass(tests)
 
     def _get_directory_containing_module(self, module_name):
diff -r 3fbfa61634de Lib/unittest/test/test_discovery.py
--- a/Lib/unittest/test/test_discovery.py	Thu Jul 19 00:14:35 2012 -0500
+++ b/Lib/unittest/test/test_discovery.py	Thu Jul 19 19:54:06 2012 -0300
@@ -180,7 +180,7 @@
         top_level_dir = os.path.abspath('/foo/bar')
         start_dir = os.path.abspath('/foo/bar/baz')
         self.assertEqual(suite, "['tests']")
-        self.assertEqual(loader._top_level_dir, top_level_dir)
+        self.assertEqual(loader._top_level_dir, None)
         self.assertEqual(_find_tests_args, [(start_dir, 'pattern')])
         self.assertIn(top_level_dir, sys.path)