diff --git a/tool/mk_call_iseq_optimized.rb b/tool/mk_call_iseq_optimized.rb
index 0760ab2667..9b88c5eeb6 100644
--- a/tool/mk_call_iseq_optimized.rb
+++ b/tool/mk_call_iseq_optimized.rb
@@ -1,5 +1,6 @@
 
 puts <<EOS
+/* -*- c -*- */
 #if 1 /* enable or disable this optimization */
 
 /* DO NOT EDIT THIS FILE DIRECTLY