Node.app - runs locally, deploys correctly but fails to work

Node.app - runs locally, deploys correctly but fails to work

i'm trying to deploy my node app on heroku.
This app works fine localy, and is properly pushed on heroku.
However, whenever I try to reach the URL, the browsers displays:
An error occurred in the application and your page could not be served.
Please try again in a few moments.
If you are the application owner, check your logs for details
At first, I thought that was because I didn't pay attention the port I was
listening to (I don't really understand how Heroku processes and if the
port is important) so I copied the same code as they did in their Getting
started...:
port = process.env.PORT || 5000;
server.listen(port, function() {
console.log("Listening on " + port);
});
my package.json is very basic :
{
"name":"Radiorev",
"version":"0.0.1",
"dependencies":{
"express":"3.3.5",
"socket.io":"0.9.16"
}
}
Help!
By the way when I type heroku log, my cmd tools displays something quite
unreadable... with zero line breaks.