NilClass

From RubySpec

According to Programming Ruby, first edition, NilClass is the "class of the singleton object nil".

Behavior of nil

irb(main):008:0> nil.nil?
=> true
irb(main):009:0> nil.to_a
=> []
irb(main):010:0> nil.to_i
=> 0
irb(main):011:0> nil.to_s
=> ""
irb(main):028:0> nil.class
=> NilClass