diff --git a/PRESUBMIT_test_mocks.py b/PRESUBMIT_test_mocks.py
index eb78f61..f8143ae 100644
--- a/PRESUBMIT_test_mocks.py
+++ b/PRESUBMIT_test_mocks.py
@@ -126,7 +126,7 @@
       if file_.LocalPath() == filename:
         return '\n'.join(file_.NewContents())
     # Otherwise, file is not in our mock API.
-    raise IOError("No such file or directory: '%s'" % filename)
+    raise IOError, "No such file or directory: '%s'" % filename
 
 
 class MockOutputApi(object):