I am trying to use nginx as a reverse proxy with upstream SSL. For this, I am using the below directive in the nginx configuration file

proxy_pass https://<upstream_block_file_name>;

where "<upstream_block_file_name>" is another file which has the list of upstream servers.

upstream <upstream_block_file_name> {
server <IP_address_of_upstream_server>:<Port> weight=1;
keepalive 100;
}

With this configuration if I try to reload the Nginx configuration, it fails intermittently with the below error message

nginx: [emerg] host not found in upstream \"<upstream_block_file_name>\"

However, if I changed the protocol mentioned in the proxy_pass directive from https to http, then the reload goes through.

Could anyone please explain what mistake I might be doing here?

Thanks in advance.