diff --git a/lib/vagrant/plugin/v2/plugin.rb b/lib/vagrant/plugin/v2/plugin.rb
index 0b191b8b7..4a680734c 100644
--- a/lib/vagrant/plugin/v2/plugin.rb
+++ b/lib/vagrant/plugin/v2/plugin.rb
@@ -1,4 +1,5 @@
 require "log4r"
+require "set"
 
 module Vagrant
   module Plugin