Nginx *2148 directory index of “/dev/shm/screenshots/” is forbidden

I have some problems with nginx with some location config, here is my config file:

user  dev-ci;

location ~* /img/screenshots/ {
    alias /dev/shm/screenshots/;
}

# other location with root and proxing to apache currently working propertly

My petition with browser is:

http://URL/img/screenshots/4336.jpg?id=85574

My nginx log is:

2012/04/23 08:49:40 [error] 13916#0: *2148 directory index of "/dev/shm/screenshots/" is forbidden, client: *.*.*.5, server: development, request: "GET /img/screenshots/4336.jpg/?id=85574 HTTP/1.1", host: "****.amazonaws.com:2000"

My ls -la from directory /dev/shm/screenshots/:

drwxrwxrwx  3 dev-ci dev-ci   80 2012-04-23 08:54 .
drwxrwxrwt 11 root   root    260 2012-04-23 07:27 ..
-rw-r--r--  1 dev-ci dev-ci 7450 2012-04-23 08:54 4336.jpg
drwxrwxrwx  2 root   root     40 2012-04-23 07:25 errors

The nginx user running is: dev-ci (here we can see the ps aux | grep nginx)

root      1231  0.0  0.0  22052  1432 ?        Ss   07:25   0:00 nginx: master process /usr/local/nginx/sbin/nginx
dev-ci   13916  0.0  0.0  22448  1760 ?        S    08:48   0:00 nginx: worker process      

Any idea why isn't working? I get 403 Forbidden error directly to nginx, so the first location is working, but it doesn't alias correctly. The nginx process has the same username as the file, the directory has 777, I tried to change the owner and group to dev-ci user with the same result. I need a bright idea, and soon…

Thanks!