Failing rio#mkdir in rio-0.3.4

This seems quite basic to me, and it worked with rio-0.3.3:

require ‘rubygems’
require ‘rio’

But with rio-0.3.4 it says:

uninitialized constant RIO::Ops::FileOrDir::ExistOrNot (NameError)

my ruby is:
ruby 1.8.4 (2005-12-24) [i686-linux]

Is there anybody on this list with:

ruby 1.8.3 or 1.8.4 and

who can run this:

require ‘rubygems’
require ‘rio’
p rio(??).mkdir.path

without getting an error message?