chapter 8: fixing the bug

While implementing a new method called debit that decrements the @balance by the given amount by

class Account
  def deposit(amount)
    @balance = amount
  end

  def balance
    @balance 
  end

  def debit(amount)
    @balance -= amount
  end
end

class Teller
  def initialize(cash_slot)    
    @cash_slot = cash_slot
  end

  def withdraw_from(account, amount)

    account.debit(amount)
    @cash_slot.dispense(amount)
  end
end

error:
undefined method `-' for nil:NilClass (NoMethodError)
      ./lib/sample_bank.rb:17:in `debit'
      ./lib/sample_bank.rb:40:in `withdraw_from'
      ./lib/sample_bank.rb:66:in `block in <top (required)>'
      ./features/support/world_extensions.rb:8:in `withdraw_from''

Did, i got mistakes somewhere else. can you explain me about that.