diff --git a/tests/__init__.py b/tests/__init__.py
index 94c97dc5f33fb3ba4da16a057d324404d5d021f9..13d510658476d98e22d1176d669b3e0e39dd5319 100644
--- a/tests/__init__.py
+++ b/tests/__init__.py
@@ -94,7 +94,7 @@ class CaptureStdout(object):
 
 
 
-def testdir(contents=None, suffix=None):
+def testdir(contents=None, suffix=""):
     """Returnsa context manager which creates, changes to, and returns a
     temporary directory, and then deletes it on exit.
     """