diff --git a/lib/rexml/xpath_parser.rb b/lib/rexml/xpath_parser.rb
index bb655f51..761b5281 100644
--- a/lib/rexml/xpath_parser.rb
+++ b/lib/rexml/xpath_parser.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: false
 
 require "pp"
+require "set"
 
 require_relative 'namespace'
 require_relative 'xmltokens'