Ruby FTP error

Dang, Ruby is giving me an FTP error, and I don't know what to do about it. Google doesn't give me much information on it:

irb(main):001:0> require 'net/ftp'
=> true
irb(main):002:0> Net::FTP.open('ftp.ruby-lang.org')
EOFError: End of file reached
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:211:in `readline'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:211:in `getline'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:221:in `getmultiline'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:235:in `getresp'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:251:in `voidresp'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:176:in `connect'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:174:in `synchronize'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:174:in `connect'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:136:in `initialize'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:120:in `new'
        from c:/ruby/lib/ruby/1.8/net/ftp.rb:120:in `open'
        from (irb):2

Update: It's the firewall. When I turn my firewall off, the error goes away.

Update: The problem somehow vanished, after a combination of reboots and reinstallations of my virus scanner (Avast). Hooray!