diff --git a/tools/diff.py b/tools/diff.py
index 29bc1ff33..1ec660e68 100644
--- a/tools/diff.py
+++ b/tools/diff.py
@@ -6,7 +6,7 @@ import difflib
 import sys
 
 diff = difflib.unified_diff(
-    open(sys.argv[1]).readlines(),
-    open(sys.argv[2]).readlines())
-with open(sys.argv[3], 'w') as f:
+    open(sys.argv[1], encoding='utf-8').readlines(),
+    open(sys.argv[2], encoding='utf-8').readlines())
+with open(sys.argv[3], 'w', encoding='utf-8') as f:
     f.write(''.join(diff))