Nginx reload intermittenlty fails when protocol specified in proxy_pass directive is specified as HTTPS

[shivramg94]

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.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,277399,277399#msg-277399