irb(main):010:0> s = 'a string!'
=> "a string!"
irb(main):011:0> s << nil
TypeError: cannot convert nil into String
	from (irb):11:in `<<'
	from (irb):11
	from :0
irb(main):012:0> s << nil.to_s
=> "a string!"
irb(main):013:0> WTF!!!!