不能将nil强制转换为BigDecimal-Ruby On Rails（rails 2.3.2）

我尝试使用以下代码。我收到“无法将nil强制转换为BigDecimal”的错误消息。任何人都可以帮助我。谢谢。

迁移文件

class AddPaidAmountToVoucher < ActiveRecord::Migration
  def self.up
    add_column :vouchers, :paid_amount, :decimal, :precision => 18, :scale => 2
  end

  def self.down
    remove_column :vouchers, :paid_amount
  end
end

模型类（voucher_payment.rb）

after_update :update_voucher_paid_amount

def update_voucher_paid_amount
    voucher = self.voucher
    paidAmount = 0
    voucher.voucher_payments.each do |voucher_payment|
      paidAmount += voucher_payment.paid_amount if voucher_payment.is_approved == true
    end
    voucher.paid_amount = paidAmount 

    voucher.save
  end