diff --git a/test/psych/visitors/test_yaml_tree.rb b/test/psych/visitors/test_yaml_tree.rb
index 6a9dbc1e..4c48670f 100644
--- a/test/psych/visitors/test_yaml_tree.rb
+++ b/test/psych/visitors/test_yaml_tree.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require 'psych/helper'
+require 'delegate'
 
 module Psych
   module Visitors