diff --git a/test/test_tsort.rb b/test/test_tsort.rb
index 4a13a6e635..354d928908 100644
--- a/test/test_tsort.rb
+++ b/test/test_tsort.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'tsort'
 require 'test/unit'