diff --git a/Lib/doctest.py b/Lib/doctest.py
index 55f15f1..979d6d0 100644
--- a/Lib/doctest.py
+++ b/Lib/doctest.py
@@ -281,6 +281,7 @@
     'DocTestSuite',
     'testsource',
     'debug',
+    'master',
 ]
 
 import __future__