diff --git a/lib/mutant.rb b/lib/mutant.rb
index 3cf79b92..39f613c3 100644
--- a/lib/mutant.rb
+++ b/lib/mutant.rb
@@ -1,5 +1,6 @@
 # encoding: utf-8
 
+require 'stringio'
 require 'benchmark'
 require 'set'
 require 'adamantium'