title: `-': nil can't be coerced into Fixnum (TypeError)
body: <p>In my program Im getting an error which says ./ruby_camping.rb:91:in `-': nil can't be coerced into Fixnum (TypeError). What I would like to do is to checkout a guest that I have just checked in. Here is the block of code for the checkout part:</p>
  
  <pre><code>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 &lt; 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
  </code></pre>
  
  <p>How come departureDate still is nil? Thankful for help!                               </p>