diff --git a/lib/rack/session/cookie.rb b/lib/rack/session/cookie.rb
index bb541396f..737c0b33b 100644
--- a/lib/rack/session/cookie.rb
+++ b/lib/rack/session/cookie.rb
@@ -5,6 +5,7 @@
 require_relative 'abstract/id'
 require 'json'
 require 'base64'
+require 'delegate'
 
 module Rack