Test code (run from the Documents directory of a standard OS X user account).

    d = Dir.new('Pictures')

    pn = Pathname.new(d)

    dir = pn.children.find { |child| child.directory? && child.basename.to_s == 'GoPro' }
    puts dir.inspect
    
JRuby 1.7.5 result:

    ‼ ⚡ ruby dir_test.rb
    TypeError: can't convert Dir into String
    initialize at org/jruby/ext/pathname/RubyPathname.java:201
          (root) at dir_test.rb:7
          
MRI 1.9.3-p392 result:

    ✔ ⚡ ruby dir_test.rb
    #<Pathname:Pictures/GoPro>
    
Work-around:

    pn = Pathname.new(d.path)