diff --git a/lib/rubocop/server/core.rb b/lib/rubocop/server/core.rb
index f00a9bceae6e..7df92d71e1f1 100644
--- a/lib/rubocop/server/core.rb
+++ b/lib/rubocop/server/core.rb
@@ -2,6 +2,7 @@
 
 require 'securerandom'
 require 'socket'
+require 'stringio'
 
 #
 # This code is based on https://github.com/fohte/rubocop-daemon.