diff --git a/benchmark/app_aobench.rb b/benchmark/app_aobench.rb
index 2bd6acfaf8..13cbc1657d 100644
--- a/benchmark/app_aobench.rb
+++ b/benchmark/app_aobench.rb
@@ -1,3 +1,5 @@
+# coding: US-ASCII
+
 # AO render benchmark
 # Original program (C) Syoyo Fujita in Javascript (and other languages)
 #      https://code.google.com/p/aobench/