diff --git a/Tools/iobench/iobench.py b/Tools/iobench/iobench.py
index b3bdd6a..5ec6f17 100644
--- a/Tools/iobench/iobench.py
+++ b/Tools/iobench/iobench.py
@@ -358,7 +358,7 @@
             with text_open(name, "r") as f:
                 return f.read()
         run_test_family(modify_tests, "b", text_files,
-            lambda fn: open(fn, "r+"), make_test_source)
+            lambda fn: text_open(fn, "r+"), make_test_source)
 
 
 def prepare_files():