Create bottles_of_beer

Bottles of beer. I'm having trouble with line 88, less_number = bottles_of_beer(number.to_i - 1).

I get the error message: bottles_of_beer.rb:88:in `+': no implicit conversion of nil into String (TypeError)
	from bottles_of_beer.rb:88:in `bottles_of_beer'
	from bottles_of_beer.rb:88:in `bottles_of_beer'
	from bottles_of_beer.rb:88:in `bottles_of_beer'
	from bottles_of_beer.rb:100:in `<main>'