diff --git a/test/test_helper.rb b/test/test_helper.rb
index 8d3a324..e600b84 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -1,4 +1,5 @@
 gem 'minitest'
+require 'set'
 require 'minitest/autorun'
 
 module TestHelper