为什么ActionMailer :: Base.deliveries.last.to未定义的方法为零？

Question

鉴于以下内容：

it "sends an email to user" do
  subscription = FactoryGirl.create(:subscription, stripe_customer_token: 'bloop')
  subscription.expire!
  ActionMailer::Base.deliveries.last.to.should == [subscription.user.email]
end

我明白了：

Failure/Error: ActionMailer::Base.deliveries.last.to.should == [subscription.user.email]
 NoMethodError: undefined method `to' for nil:NilClass
 # ./spec/models/subscription_spec.rb:113:in `block (3 levels) in <top (required)>'

我不明白，因为如果我在控制台的开发中做同样的事情，我会看到发送的电子邮件。我有test.rb config.action_mailer.delivery_method = :test

问题是什么？