date_select issue in Rails

I have implemented date_select in my views when i access it in to model it gives me error:

View:

<%= date_select :transaction ,:card_expires_on,:discard_day => true, :start_year => Date.today.year, :end_year => (Date.today.year+10), :add_month_numbers => true, :order => [:month,:year]%>

Model:

 def credit_card
    @credit_card ||= ActiveMerchant::Billing开发者_如何学运维::CreditCard.new(
      :type               => card_type,
      :number             => card_number,
      :verification_value => card_verification,
      :month              => card_expires_on.month,
      :year               => card_expires_on.year,
      :first_name         => first_name,
      :last_name          => last_name
    )
  end

error:

You have a nil object when you didn't expect it!
The error occurred while evaluating nil.month

Based on what you've posted, I'd say that card_expires_on in your credit_card method is nil.