diff --git a/lib/wayback_machine_downloader.rb b/lib/wayback_machine_downloader.rb
index c360b1e..f58e9a8 100644
--- a/lib/wayback_machine_downloader.rb
+++ b/lib/wayback_machine_downloader.rb
@@ -1,5 +1,6 @@
 # encoding: UTF-8
 
+require 'thread'
 require 'net/http'
 require 'open-uri'
 require 'fileutils'