上游节点服务器关闭与Nginx的连接

我将nginx用作速率限制请求的节点服务器的代理。速率是每30秒请求一次；大多数请求返回的响应都很好，但是如果请求长时间保持打开状态，我会得到以下信息：

upstream prematurely closed connection while reading response header from upstream

我不知道是什么原因造成的。下面是我的nginx配置：

# For more information on configuration, see:
#   * Official English Documentation: http://nginx.org/en/docs/
#   * Official Russian Documentation: http://nginx.org/ru/docs/

user nginx;
worker_processes auto;
error_log /var/log/nginx/error.log;
pid /run/nginx.pid;

# Load dynamic modules. See /usr/share/nginx/README.dynamic.
# include /usr/share/nginx/modules/*.conf;

events {
    worker_connections 1024;
}

http {
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile            on;
    tcp_nopush          on;
    tcp_nodelay         on;
    keepalive_timeout   65;
    types_hash_max_size 2048;

    include             /etc/nginx/mime.types;
    default_type        application/octet-stream;

    # Load modular configuration files from the /etc/nginx/conf.d directory.
    # See http://nginx.org/en/docs/ngx_core_module.html#include
    # for more information.
    include /etc/nginx/conf.d/*.conf;

    server {
        listen       80 default_server;
        listen       [::]:80 default_server;
        server_name  _;
        root         /srv/www/main/htdocs;

        # Load configuration files for the default server block.
        include /etc/nginx/default.d/*.conf;    

        location /vcheck {
            proxy_pass http://127.0.0.1:8080$is_args$query_string;
            # proxy_buffer_size 128k;
            # proxy_buffers 4 256k;
            # proxy_busy_buffers_size 256k;
            # proxy_http_version 1.1;
            # proxy_set_header Upgrade $http_upgrade;
            # proxy_set_header Connection 'upgrade';
            # proxy_set_header Host $host;
            # proxy_cache_bypass $http_upgrade;         
            # proxy_redirect off;

            proxy_read_timeout 600s;
        }

        location ~ \.php$ {
            include fastcgi.conf;
            fastcgi_split_path_info ^(.+\.php)(/.+)$;
            fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock;
            fastcgi_index routes.php$is_args$query_string;
        }

        location / {
            if (-f $request_filename) {
                expires max;
                break;
            }

            if ($request_filename !~ "\.(js|htc|ico|gif|jpg|png|css)$") {
                rewrite ^(.*) /routes.php last;
            }
        }       

    }
}

为什么Node可能会提早关闭连接？

编辑：我正在使用Node的内置HTTP服务器。