diff --git a/activemodel/lib/active_model/validations/length.rb b/activemodel/lib/active_model/validations/length.rb
index eb7aac709d..6bc928bab7 100644
--- a/activemodel/lib/active_model/validations/length.rb
+++ b/activemodel/lib/active_model/validations/length.rb
@@ -23,7 +23,7 @@ module ActiveModel
         keys = CHECKS.keys & options.keys
 
         if keys.empty?
-          raise ArgumentError, 'Range unspecified. Specify the :within, :maximum, :minimum, or :is option.'
+          raise ArgumentError, 'Range unspecified. Specify the :in, :within, :maximum, :minimum, or :is option.'
         end
 
         keys.each do |key|