diff --git a/test/ripper/test_ripper.rb b/test/ripper/test_ripper.rb
index 13a5294618..ab841b7616 100644
--- a/test/ripper/test_ripper.rb
+++ b/test/ripper/test_ripper.rb
@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 begin
   require 'ripper'
+  require 'stringio'
   require 'test/unit'
   ripper_test = true
   module TestRipper; end