assigning to nil as an array

gistfile1.txt

15:23:14 tims@firebat ~ $ rvm use 2.2.1
Using /home/tims/.rvm/gems/ruby-2.2.1
15:23:18 tims@firebat ~ $ irb
2.2.1 :001 > "#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}"
 => "2.2.1-p85" 
2.2.1 :002 > a = nil
 => nil 
2.2.1 :003 > a[5] = 6
NoMethodError: undefined method `[]=' for nil:NilClass
        from (irb):3
        from /home/tims/.rvm/rubies/ruby-2.2.1/bin/irb:11:in `<main>'
2.2.1 :004 > a[5]
NoMethodError: undefined method `[]' for nil:NilClass
        from (irb):4
        from /home/tims/.rvm/rubies/ruby-2.2.1/bin/irb:11:in `<main>'
2.2.1 :005 > 
15:23:38 tims@firebat ~ $ rvm use 2.3.0
Using /home/tims/.rvm/gems/ruby-2.3.0
15:23:52 tims@firebat ~ $ irb
2.3.0 :001 > "#{RUBY_VERSION}-p#{RUBY_PATCHLEVEL}"
 => "2.3.0-p0" 
2.3.0 :002 > a = nil
 => nil 
2.3.0 :003 > a[5] = 6
 => nil 
2.3.0 :004 > a[5]
NoMethodError: undefined method `[]' for nil:NilClass
        from (irb):4
        from /home/tims/.rvm/rubies/ruby-2.3.0/bin/irb:11:in `<main>'
2.3.0 :005 >