diff --git a/db/migrate/20220415193829_add_index_for_question_id_index_on_inboxes.rb b/db/migrate/20220415193829_add_index_for_question_id_index_on_inboxes.rb
index 63ef74c7..884d1650 100644
--- a/db/migrate/20220415193829_add_index_for_question_id_index_on_inboxes.rb
+++ b/db/migrate/20220415193829_add_index_for_question_id_index_on_inboxes.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class AddIndexForQuestionIdIndexOnInboxes < ActiveRecord::Migration[6.1]
   def change
     add_index :inboxes, :question_id