diff --git a/lib/active_flag/value.rb b/lib/active_flag/value.rb
index cc0f40d..e5269ff 100644
--- a/lib/active_flag/value.rb
+++ b/lib/active_flag/value.rb
@@ -1,3 +1,5 @@
+require 'set'
+
 module ActiveFlag
   class Value < Set
     def with(instance, definition)