Module#const_defined? weirdness in Ruby 1.9.3

gistfile1.rb

mod = Module.new do
  Junk = 123
end
mod.const_defined?(:Junk) #=> false
mod.const_defined?(:Junk, false) #=> ArgumentError
mod.constants #=> []

module Foo
  Junk = 123
end
Foo.const_defined?(:Junk) #=> true
Foo.const_defined?(:Junk, false) #=> ArgumentError
Foo.constants #=> ["Junk"]