diff --git a/lib/locale_file_walker.rb b/lib/locale_file_walker.rb
index 0ac3146e8..4f11ea9ff 100644
--- a/lib/locale_file_walker.rb
+++ b/lib/locale_file_walker.rb
@@ -1,4 +1,5 @@
 require 'psych'
+require 'set'
 
 class LocaleFileWalker
   protected