Nginx autoindex not working - 404 error

I have a server block which displays properly, unless I set it to autoindex. This results in a 404 not found error.

server block:

<code> server {
# listen 80;
# server_name $domain_name;
root /var/www/html/files;
# root /var/www/html;
# autoindex on;
access_log /var/log/nginx/access.log;
error_log /var/log/nginx/error.log;
}
</code>