diff --git a/config.ru b/config.ru
index 80b34739f01b5c82e3a58d8d639a04f9652da30f..e65bf31ea96bb5df149e2c09d58cbbf1e11d0f32 100644
--- a/config.ru
+++ b/config.ru
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "braintree"
 require "delegate"
 require "dhall"