diff --git a/lib/vagrant/plugin/v1/plugin.rb b/lib/vagrant/plugin/v1/plugin.rb
index 301ad98e0..81afcc4d1 100644
--- a/lib/vagrant/plugin/v1/plugin.rb
+++ b/lib/vagrant/plugin/v1/plugin.rb
@@ -1,3 +1,5 @@
+require "set"
+
 require "log4r"
 
 module Vagrant