ruby – ` – ‘：nil不能被强制转换为Fixnum(TypeError)

在我的程序中我得到一个错误,上面写着./ruby_camping.rb：91：in` – ‘：nil无法强制进入Fixnum(TypeError).我想做的是结账我刚刚签到的客人.以下是结账部分的代码块：

def self.check_out 
  puts "Welcome to checkout!"
  puts $camping.current_guests
  puts " Here are the current guests,who do you want to checkout?!" 
  puts "State plot number "
  plot = gets.chomp.to_i
  guest = $camping.current_guests[plot-1] 
  puts "State the date for your checkout (a number bigger then " + guest.arrived.to_s + "): "
  # it should not be possible for a guest to checkout before she has checked in
  departureDate = gets.chomp.to_i
  guestStayedDays = departureDate - guest.arrived #Days the guest has stayed
  while (guestStayedDays < 1) do                        
   puts "The date for checkout is not valid. The guest checked in " + guest.arrived.to_s
   puts "Please state a new date."
   departureDate = gets.chomp.to_i
   guestStayedDays = departureDate - guest.arrived
  end       
  guest.departure = departureDate
  guest.plot.increase(guestStayedDays) # increases the electricity meter
  puts guest # prints the guest
  $camping.current_guests[plot-1] = nil # emptys the plot
 end
end

为什么离开日期仍然是零？感谢您的帮助！

irb(main):001:0> 5 - nil
TypeError: nil can't be coerced into Fixnum
        from (irb):1:in `-'
        from (irb):1