Weirdness with defined? on method chains

defined.rb

class Foo
  def method_missing(name, *args)
    puts "in #{name}"
    self
  end

  def respond_to_missing?(name, include_private)
    true
  end
end

k = Foo.new

puts defined?(k.a)
puts defined?(k.a.b)
puts defined?(k.a.b.c)
puts defined?(k.a.b.c.d)
puts defined?(k.a.b.c.d.e)
puts defined?(k.a.b.c.d.e.f)

output

➜ ruby --version
ruby 1.9.2p290 (2011-07-09 revision 32553) [x86_64-darwin10.8.0]
➜ ruby defined.rb 
method
in a
method
in a
in a
in b
method
in a
in a
in b
in a
in b
in c
method
in a
in a
in b
in a
in b
in c
in a
in b
in c
in d
method
in a
in a
in b
in a
in b
in c
in a
in b
in c
in d
in a
in b
in c
in d
in e
method