From my OSX box I get

/usr/lib/ruby/1.6/net/ftp.rb:175:in `addr': getnameinfo: nodename nor 
servname provided, or not known (SocketError)
        from /usr/lib/ruby/1.6/net/ftp.rb:175:in `makeport'
        from /usr/lib/ruby/1.6/net/ftp.rb:209:in `transfercmd'
        from /usr/lib/ruby/1.6/net/ftp.rb:286:in `retrlines'
        from /usr/lib/ruby/1.6/net/ftp.rb:285:in `mon_synchronize'
        from /usr/lib/ruby/1.6/net/ftp.rb:285:in `retrlines'
        from /usr/lib/ruby/1.6/net/ftp.rb:447:in `list'
        from rt:6

On my Linux box I get

/usr/lib/ruby/1.6/net/ftp.rb:129:in `getresp': 500 Illegal PORT command. 
(Net::FTPPermError)

        from /usr/lib/ruby/1.6/net/ftp.rb:137:in `voidresp'
        from /usr/lib/ruby/1.6/net/ftp.rb:154:in `voidcmd'
        from /usr/lib/ruby/1.6/net/ftp.rb:153:in `mon_synchronize'
        from /usr/lib/ruby/1.6/net/ftp.rb:153:in `voidcmd'
        from /usr/lib/ruby/1.6/net/ftp.rb:170:in `sendport'
        from /usr/lib/ruby/1.6/net/ftp.rb:178:in `makeport'
        from /usr/lib/ruby/1.6/net/ftp.rb:209:in `transfercmd'
        from /usr/lib/ruby/1.6/net/ftp.rb:286:in `retrlines'
        from /usr/lib/ruby/1.6/net/ftp.rb:285:in `mon_synchronize'
        from /usr/lib/ruby/1.6/net/ftp.rb:285:in `retrlines'
        from /usr/lib/ruby/1.6/net/ftp.rb:447:in `list'
        from rt:6

maybe it is not an OSX error?