Index: test/lib/TestCmd.py
--- test/lib/TestCmd.py.orig
+++ test/lib/TestCmd.py
@@ -882,7 +882,7 @@ class TestCmd(object):
                 #self.diff_function = difflib.unified_diff
         self._dirlist = []
         self._preserve = {'pass_test': 0, 'fail_test': 0, 'no_result': 0}
-        if 'PRESERVE' in os.environ and os.environ['PRESERVE'] is not '':
+        if 'PRESERVE' in os.environ and os.environ['PRESERVE'] != '':
             self._preserve['pass_test'] = os.environ['PRESERVE']
             self._preserve['fail_test'] = os.environ['PRESERVE']
             self._preserve['no_result'] = os.environ['PRESERVE']