反向代理错误。 去掉proxy_set_header Host $host;就不报错了。什么问题？

反向代理错误。前一个报404，后一个报400 Request Header Or Cookie To

location =/app/index1.php
     {
           proxy_set_header Host $host;
           proxy_set_header X-Real-IP $remote_addr;
           proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
           proxy_set_header REMOTE-HOST $remote_addr; 
           proxy_set_header X-Forwarded-Proto https;
           if ($args ~ m=test) {
                proxy_pass http://dp.test.com;
           }
         	   proxy_pass http://main.test.com; 
       
       }

去掉 proxy_set_header Host $host;就不报错了什么问题

location =/app/index1.php
     {

           proxy_set_header X-Real-IP $remote_addr;
           proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
           proxy_set_header REMOTE-HOST $remote_addr; 
           proxy_set_header X-Forwarded-Proto https;
           if ($args ~ m=test) {
                proxy_pass http://dp.test.com;
           }
         	   proxy_pass http://main.test.com; 
       
       }