为Golang设置Nginx FastCGI获得403

I'm trying to use nginx as a proxy for a FastCGI go-app. The server serves the regular html-files without problems. However when going to my app directory I receive an 403 error. The error log says

    2014/03/11 09:24:27 [error] 14418#0: *3 directory index of "/usr/share/nginx/html/app/" is forbidden, client: 127.0.0.1, server: localhost, request: "GET /app/ HTTP/1.1", host: "localhost"

My nginx.conf is as follows

    server {
    listen 80;
    root /usr/share/nginx/html;
    index index.html;

    location / {
        try_files $uri $uri/;
    }

    location ~ /app.* {
        autoindex       on;
        include         fastcgi_params;
        fastcgi_pass    127.0.0.1:9000;
    }

    try_files $uri $uri.html =404;
}

in the html/app directory the executable is called app and is listening to port 9000. I've set the owner of the app directory and app file to be www-data. And nginx has read, write and execute permissions in the folder.

Any help is greatly appreciated.