Nginx wont start after reboot Ubuntu 14.04

I'm getting this error:

nginx: [emerg] duplicate upstream "app_server" in /etc/nginx/sites-enabled/django.save:1

this is my config file:

upstream app_server {
    server 127.0.0.1:9000 fail_timeout=0;
}

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    root /usr/share/nginx/html;
    index index.html index.htm;

    client_max_body_size 4G;
    server_name _;

    keepalive_timeout 5;

    # Your Django project's media files - amend as required
    location /media  {
        alias /home/django/django_project/django_project/media;
    }

    # your Django project's static files - amend as required
    location /static {
        alias /home/django/django_project/django_project/static; 
    }

    location / {
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://app_server;
    }
}