nginx reverse server upstream name syntax

I very appreciated if someone can give me some help. I just tried to learn the nginx reverse proxy but have experienced some problem about upstream name syntax.

Please refer to this photo Photo: nginx reverse proxy server structure

When I curl 192.168.189.140, the proxy server shows

1. [debug] vhost.c(811): [client 192.168.189.140] [strict] Invalid host name 'bbs_server_pools', problem near: _serve

2. [debug] vhost.c(902): [client 192.168.189.140] Client sent malformed Host header: bbs_server_pools

After I changed upstream name bbs_server_pools to bbs, it works all good. Is this something to do with the version of nginx or something else?

1. error Nginx.conf file

   worker_processes  1;
   error_log logs/error.log error;
   
   events {
   worker_connections  1024;
   }
   
   http {
   include       mime.types;
   default_type  application/octet-stream;
   sendfile        on;
   keepalive_timeout  65;
   log_format main '$remote_addr - $remote_user [$time_local] "$request"'
                   '$status $body_bytes_sent "$http_referer"'
                   '"$http_user_agent" "$http_x_forwarded_for"';
   
   server {
   listen       80;
   server_name  bbs.etiantian.org;
   location / {
        proxy_pass http://bbs_server_pools;   
      } 
   }    
   
   upstream bbs_server_pools {
   server 192.168.189.137:80;
   server 192.168.189.138:80 weight=3;
   }  
   
   
   }
   

2. good nginx.conf file after changing bbs_server_pools to bbs

   worker_processes  1;
   error_log logs/error.log error;
   
   events {
   worker_connections  1024;
   }
   
   http {
   include       mime.types;
   default_type  application/octet-stream;
   sendfile        on;
   keepalive_timeout  65;
   log_format main '$remote_addr - $remote_user [$time_local] "$request"'
                   '$status $body_bytes_sent "$http_referer"'
                   '"$http_user_agent" "$http_x_forwarded_for"';
   
   server {
   listen       80;
   server_name  bbs.etiantian.org;
   location / {
        proxy_pass http://bbs;   
      } 
   }    
   
   upstream bbs {
   server 192.168.189.137:80;
   server 192.168.189.138:80 weight=3;
   }  
   
   
   }