diff --git a/lib/validates_timeliness/validator.rb b/lib/validates_timeliness/validator.rb
index b52e9797..910be1c3 100644
--- a/lib/validates_timeliness/validator.rb
+++ b/lib/validates_timeliness/validator.rb
@@ -1,3 +1,4 @@
+require 'active_model'
 require 'active_model/validator'
 
 module ValidatesTimeliness