diff --git a/build.py b/build.py
index 01b166ab39d2ca621d6dffdaf257e1a17bf787f5..111d6da9d7c8a3c6712a530c1836e38b23f7b226 100644
--- a/build.py
+++ b/build.py
@@ -3,7 +3,7 @@ from __future__ import print_function
 import json, sys
 import os.path
 
-def readin(path, func=lambda x: x, orelse=None):
+def readin(path, func=bytes, orelse=None):
     if os.path.exists(path):
         with open(path, 'rb') as fin:
             return func(fin.read())