diff --git a/lib/mutant/killer/rspec.rb b/lib/mutant/killer/rspec.rb
index a7fc2a8a..80c2c901 100644
--- a/lib/mutant/killer/rspec.rb
+++ b/lib/mutant/killer/rspec.rb
@@ -18,6 +18,7 @@ module Mutant
       def run
         mutation.insert
         # TODO: replace with real streams from configuration
+        require 'stringio'
         null = StringIO.new
         !::RSpec::Core::Runner.run(command_line_arguments, null, null).zero?
       end