"translation missing" when using validator on non-activerecord backed models

[ghiculescu]

I have a model that's not an ActiveRecord subclass:

class Demo
  include ActiveModel::Validations
  include ActiveModel::Conversion
  extend ActiveModel::Naming

  def initialize(attributes = {})
    attributes.each do |name, value|
      send("#{name}=", value)
    end
  end

  def persisted?
    false
  end

  attr_accessor :phone
  validates_plausible_phone :phone
end

It has this validator:

validates :phone, :phony_plausible => true

But I've also tried:

validates_plausible_phone :phone

Either way, when I attempt to save with an invalid phone number, the validation message displayed to the user is:

Phone translation missing: en.activemodel.errors.models.demo.attributes.phone.improbable_phone

If I use the validators on an ActiveRecord-backed model:

class User < ActiveRecord::Base
  attr_accessible :phone 
  validates_plausible_phone :phone
end

I get the correct validation message:

Phone is an invalid number

I'm happy to manually add the translation but I was just wondering if this is expected?