Failing site_prism / capybara test reduced to the relevant parts

client_paydata_section.rb

# frozen_string_literal: true

##
# Client paydata section containing the client billing address (removed) and the payment method as subsections.
#
class ClientPaydataSection < SitePrism::Section
  class ClientPaymentMethodSection < SitePrism::Section
    element :show, ".payment-method-section .show-layer"
    element :edit_button, "a#edit_payment_method_button"
    element :edit_form, ".payment-method-section .edit-layer"

    element :credit_card_radio_button, "#credit_card_radio_button"
    # ... unrelevant elements removed

    # CC Form elements. Must be defined as iframe otherwise no access to fields!
    class CcIframe < SitePrism::Page
      # ... unrelevant elements inside ifame removed

      def update_to_credit_card(credit_card)
        # should set some values and click, but is never reached
      end
    end

    iframe :cc_iframe, CcIframe, "#credit_card_data"

    def update_to_credit_card(credit_card)
      wait_for_cc_iframe
      cc_iframe do |iframe|                        # <-- this is where the error happens, original line: 138
        iframe.update_to_credit_card(credit_card)
      end
    end
  end

  section :payment_method, ClientPaymentMethodSection, ".payment-method-section"
end

client_paydata_test_page.rb

# frozen_string_literal: true

class ClientPaydataTestPage < SitePrism::Page
  # ... unrelevant elements removed
  section :client_paydata, ClientPaydataSection, "#client-paydata-control"

  def load_page(options = {})
    load(query: { embedded_by: "atest", test_action_allowed: true }.merge(Hash(options[:query])))
  end
end

test_sale_spec.rb

feature "Offline sale", "" do
  Steps "something with the iframe form" do
    Given "A user creates a client with payment method credit card" do
      # ... removed: user will be created
      embedding_page = ClientPaydataTestPage.new
      embedding_page.load_page
      # ... removed: client will be created
      expect(embedding_page.client_paydata.payment_method.edit_form).to be_visible
      embedding_page.client_paydata.payment_method.credit_card_radio_button.click
      embedding_page.client_paydata.payment_method.update_to_credit_card(credit_card) # <-- error happens inside this
      # ... removed: rest
    end
    # ... removed: rest of test which is never reached
  end
end