Trying to use virtual collections inside of a virtual collection.

auto.rb

# Required for anonymous struct of cross_sell products.
require 'disposable/twin/struct'  # Disposible version 0.4.3.

# Contract for validating input for sending an auto email.
class Email::ThankYou::Auto::Contract < Reform::Form # Reform version: 2.2.4
  # Included for cross_sell to construct itself.
  include Disposable::Twin::Property::Struct
  
  property :username, virtual: true
  validates :username, presence: true

  property :password, virtual: true
  validates :password, presence: true

  # Tried replacing "Struct" with "OpenStruct", "Array", "Hash", etc.
  collection :cross_sell, virtual: true, field: :hash, populate_if_empty: Struct do
    property :product, virtual: true
    property :enabled, virtual: true
    property :url, virtual: true
  end
end

thank_you_auto_test.rb

require 'test_helper'

class EmailContractThankYouAutoTest < MiniTest::Spec
  describe 'Fields' do
    let (:fields) {
      {
        username: 'customer123',
        password: 'SecurePassword',
        cross_sell: [
          {
            product: 'Product 1',
            enabled: true,
            url: 'https://example.com/prod1/'
          },
          {
            product: 'Product 2',
            enabled: false,
            url: 'https://example.com/prod2/'
          }
        ]
      }
    }

    it 'requires all of the fields' do
      result = Email::ThankYou::Auto::Contract.new({}).call(fields)
      # Error:
      # Fields#test_0001_requires all of the fields:
      # NoMethodError: undefined method `original' for nil:NilClass
      result.valid?.must_equal true
    end
  end
end