diff --git a/activesupport/lib/active_support/dependencies.rb b/activesupport/lib/active_support/dependencies.rb
index 2badad5f5f..6148e59207 100644
--- a/activesupport/lib/active_support/dependencies.rb
+++ b/activesupport/lib/active_support/dependencies.rb
@@ -1,3 +1,5 @@
+require 'set'
+
 module ActiveSupport #:nodoc:
   module Dependencies #:nodoc:
     extend self