"no implicit conversion of nil into String" while specifying --port

[pvelder]

When starting parade without specifying a port, the server start fine: parade serve

-------------------------

  Your Parade presentation is now starting up.

  To view it visit [ http://localhost:9090 ]
  To view a printable version [ http://localhost:9090/print ]

-------------------------

[2013-08-23 10:05:26] INFO  WEBrick 1.3.1
[2013-08-23 10:05:26] INFO  ruby 2.0.0 (2013-06-27) [x86_64-linux]
== Sinatra/1.4.3 has taken the stage on 9090 for production with backup from WEBrick
[2013-08-23 10:05:26] INFO  WEBrick::HTTPServer#start: pid=16256 port=9090

But when starting the server, on a specific port, there is a conversion error and the Webrick server exits: parade serve --port=9091

-------------------------

  Your Parade presentation is now starting up.

  To view it visit [ http://localhost:9091 ]
  To view a printable version [ http://localhost:9091/print ]

-------------------------

error: no implicit conversion of nil into String