irb(main):001:0> def a(b = {})
irb(main):002:1> puts b[:one]
irb(main):003:1> end
=> :a
irb(main):004:0> a

=> nil
irb(main):005:0> a(nil)
NoMethodError: undefined method `[]' for nil:NilClass
	from (irb):2:in `a'
	from (irb):5
	from /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5.2/lib/rails/commands/console.rb:110:in `start'
	from /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5.2/lib/rails/commands/console.rb:9:in `start'
	from /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5.2/lib/rails/commands/commands_tasks.rb:68:in `console'
	from /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5.2/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
	from /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5.2/lib/rails/commands.rb:17:in `<top (required)>'
	from /app/bin/rails:26:in `require'
	from /app/bin/rails:26:in `<main>'