Configure Nginx Proxy_Pass for Node.js HTTP Server via UNIX Socket

I am trying to configure a Nginx server to connect to a Node.js HTTP server via a UNIX domain socket.

The Nginx configuration file:

server {
  listen 80;

  location / {
    proxy_pass http://unix:/tmp/app.socket:/;
  }
}

(according to http://wiki.nginx.org/HttpProxyModule#proxy_pass)

The Node.js script:

var http = require('http');

http.createServer(function(req, res) {
  console.log('received request');
  req.end('received request\n');
}).listen('/tmp/app.socket');

Now, when I try to call

curl http://localhost/

I only get the 502 Bad Gateway error page in curl and nothing on the Node.js process.

Am I doing something wrong?

edit:

After trying quanta's solution, the mistake must have to do with the Nginx configuration, since the Node.js process establishes the connection to the socket correctly.

I also tried to configure Nginx this way:

upstream myapp {
  server unix:/tmp/app.socket;
}

server {
  listen 80;

  location / {
    proxy_pass http://myapp;
  }
}

But this didn't work either.

BTW I'm using Nginx v1.0.6.

The following is being written to the error log in Nginx, when I use the second configuration

2011/09/28 13:33:47 [crit] 1849#0: *5 connect() to unix:/tmp/app.socket failed  (13: Permission denied) while connecting to upstream, client: 127.0.0.1,        server: , request: "GET / HTTP/1.1", upstream: "http://unix:/tmp/app.socket:/", host: "localhost:80"