fields_for, formtastic, ActiveMerchant, and validation errors

Question

I'm trying to use Formtastic to create a payment form, since I'd like to use the inline errors. I'm using ActiveMerchant to handle the billing. I have the following form:

<%= semantic_form_for @payment do %>
  <%= form.inputs do %>
    <%= form.input :email, :label => "Email Address" %>

    <%= form.semantic_fields_for :credit_card_attributes do |cc| %>
      <%= cc.input :number, :label => "Credit Card Number" %>  
      <%= cc.input :first_name, :label => "First Name" %>
      <%= cc.input :last_name, :label => "Last Name" %>
      <%= cc.input :month, :label => "Expiration Month" %>
      <%= cc.input :year, :label => "Expiration Year" %>
      <%= cc.input :verification_value, :label => "Verification Code" %>
    <% end %>
  <% end %>
<% end %>

And this is what's in my Payment model:

class Payment < ActiveRecord::Base
  validates_associated :credit_card, :on => :create

  def credit_card_attributes=(attrs)
    @credit_card = ActiveMerchant::Billing::CreditCard.new(attrs)
  end

  def credit_card
    @credit_card
  end
end

When I submit an invalid credit card, it figures out that it's invalid, but I don't get any of the inline errors from formtastic.

I figure there's probably something simple I'm missing here, I'm just not sure what.

This is on Rails 3.