diff --git a/lib/vagrant/machine_index.rb b/lib/vagrant/machine_index.rb
index e396eb70b..e19b09cd0 100644
--- a/lib/vagrant/machine_index.rb
+++ b/lib/vagrant/machine_index.rb
@@ -43,7 +43,7 @@ module Vagrant
     def initialize(data_dir)
       @data_dir   = data_dir
       @index_file = data_dir.join("index")
-      @lock       = Mutex.new
+      @lock       = Monitor.new
       @machines  = {}
       @machine_locks = {}