Как вернуть cors при 503 статусе в nginx

Необходимо установить определенные CORS к 503 ответу в nginx. Пробую так но увы не работает:

   listen 80;
   listen 443 ssl http2;
   server_name www.example.com;
   root www.example.com/public;
   index index.php index.html;

   # redirect from http to https
   if ($ssl_protocol = "") {
       rewrite ^/(.*) https://$server_name/$1 permanent;
   }

   ssl_certificate example.crt;
   ssl_certificate_key example.key;

   ssl_protocols TLSv1 TLSv1.1 TLSv1.2;

   location / {
       try_files $uri $uri/ /index.html /index.php?$query_string;
       error_page 503 /index.html;
       location = /index.html {
           root www.example.com/public;
           expires -1;
       }
       if (-f $document_root/index.html) { return 503; }
       if ($status = 503) {
            add_header 'Access-Control-Allow-Credentials' 'true' always;
       }

   }

   location ~ \.php$ {
       fastcgi_split_path_info ^(.+\.php)(/.+)$;
       fastcgi_pass 127.0.0.1:9000;
       fastcgi_index index.php;
       include fastcgi_params;
       fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
   }

}