Bug? Invalid nginx upstream slow_start parameter

problem description

I have a back-end service that takes time to start, and it takes about 5 minutes from process startup to normal access. The slow_start parameter of the nginx reverse proxy can be buffered for a period of time after the back-end service is healthy to solve the access failure after my back-end service is restarted

the environmental background of the problems and what methods you have tried

through actual testing, it is found that multiple versions of nginx cannot recognize this parameter
nginx versions that have been tested include 1.4.6,1.12,1.14

nginx: configuration file nginx/ usr/sbin/nginx-t
nginx: [emerg] invalid parameter "slow_start=300s" in/ etc/nginx/nginx.conf:59
nginx: configuration file / etc/nginx/nginx.conf test failed

related codes

/ / Please paste the code text below (do not replace the code with pictures)
Nginx upstram section is configured as follows:

upstream erp {

keepalive 1000;
server erp1.server.com:8069 weight=5 max_fails=2 fail_timeout=10s slow_start=300s;
server test1.server.com:8069 weight=5 max_fails=2 fail_timeout=10s slow_start=300s;

}

server {

listen 80;
server_name mytest.com;

client_max_body_size 100M;

access_log /var/log/nginx/erp.log;
error_log /var/log/nginx/erp.err;

location / {
    proxy_next_upstream error timeout invalid_header http_500 http_502 http_504;
    proxy_connect_timeout 3;
    proxy_read_timeout 900;
    proxy_set_header Host $http_host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-Proto $scheme;
    proxy_set_header X-Forwarded-For $remote_addr;
    proxy_pass http://erp;
}

}

what result do you expect? What is the error message actually seen?

according to the nginx official Doc display, you should be able to identify slow_start parameters normally.
and the configuration file under the upstream field is always in the official format, so you can"t find the error with the naked eye.

I don"t understand why it can"t be solved by this parameter!

slow_start=300s nginx / usr/sbin/nginx-t
nginx: [emerg] invalid parameter "slow_start=300s" in