OpenSSL pkcs7 add signer error in Rails 5, working through PayPal Security RailsCast

Question

I have been setting up a shopping cart and checkout by working through the series of RailsCasts(#141-143) and am pretty new to using OpenSSL and pkcs7. Everything has been working perfectly up until implementing security measures in (episode 143: http://railscasts.com/episodes/143-paypal-security). I keep getting a "pkcs7 add signer error"

My paypal_encripted encrypt_for_paypal methods are as follows:

def paypal_encrypted(return_url, notify_url)
    values = {
    :business => 'merchant@christianorourke.com',
    :cmd => '_cart',
    :upload => 1,
    :return => return_url,
    :invoice => id,
    :notify_url => notify_url,
    :cert_id => "ABKW58VTNJ9LY"
  }
  order_items.each_with_index do |item, index|
    values.merge!({
      "amount_#{index+1}" => item.unit_price,
      "item_name_#{index+1}" => item.product.name,
      "item_number_#{index+1}" => item.id,
      "quantity_#{index+1}" => item.quantity
    })
    end
    encrypt_for_paypal(values)
  end

PAYPAL_CERT_PEM = File.read("#{Rails.root}/certs/paypal_cert.pem")
APP_CERT_PEM = File.read("#{Rails.root}/certs/app_cert.pem")
APP_KEY_PEM = File.read("#{Rails.root}/certs/app_key.pem")

def encrypt_for_paypal(values)
  signed = OpenSSL::PKCS7::sign(OpenSSL::X509::Certificate.new(APP_CERT_PEM), OpenSSL::PKey::RSA.new(APP_KEY_PEM, ''), values.map { |k, v| "#{k}=#{v}" }.join("\n"), [], OpenSSL::PKCS7::BINARY)
  OpenSSL::PKCS7::encrypt([OpenSSL::X509::Certificate.new(PAYPAL_CERT_PEM)], signed.to_der, OpenSSL::Cipher::Cipher::new("DES3"), OpenSSL::PKCS7::BINARY).to_s.gsub("\n", "")
end

I am really unsure as to what is going wrong so any help would be greatly appreciated.