Custom forced 503 page does not work

[Dennis Jacobfeuerborn]

Hi,
I'm trying to deliver a custom 503 page for all requests the server 
receives but with the following config it doesn't work.

server {
     listen       81;
     server_name  localhost;
     error_log /var/log/nginx/error.log debug;

     root /usr/share/nginx/html;

     location / {
	return 503;
     }

     error_page   500 502 503 504  /50x.html;
     location = /50x.html {
         root   /usr/share/nginx/html;
     }
}

I get a 503 response but the 50x.html page is not used even though it is 
present in /usr/share/nginx/html/50x.html. I see no error in the error 
log even in debug mode.
What is wrong with this config?

Regards,
   Dennis