diff --git a/lib/acts_as_indexed/storage.rb b/lib/acts_as_indexed/storage.rb
index 781f164..0dfba86 100644
--- a/lib/acts_as_indexed/storage.rb
+++ b/lib/acts_as_indexed/storage.rb
@@ -1,3 +1,5 @@
+require 'thread'
+
 module ActsAsIndexed #:nodoc:
   class Storage