diff --git a/activemodel/lib/active_model/secure_password.rb b/activemodel/lib/active_model/secure_password.rb
index 826e89bf9d..0c3ed9e8ca 100644
--- a/activemodel/lib/active_model/secure_password.rb
+++ b/activemodel/lib/active_model/secure_password.rb
@@ -68,6 +68,7 @@ module ActiveModel
           validates_confirmation_of :password, if: ->{ password.present? }
         end
 
+        # This code is necessary as long as the protected_attributes gem is supported.
         if respond_to?(:attributes_protected_by_default)
           def self.attributes_protected_by_default #:nodoc:
             super + ['password_digest']