nginx proxy_pass doesn't work right when concatenating to variable - nginx

Question

I have an NGINX server running as a reverse proxy for a service I host. I tried configuring it with this:
set $upstream_nzbget http://nzbget:6789;
location /nzbget/api/ {
proxy_pass $upstream_nzbget/;
}
But the proxy_pass doesn't work properly when I try to add the slash / to the end of $upstream_nzbget. If I change it to this, it works properly:
location /nzbget/api/ {
proxy_pass http://nzbget:6789/;
}
So I think it has something to do with variables in proxy_pass. In the first case (the one that isn't working), I'm not really sure what the URL ends up being. I haven't found a way to log the final URI that gets used by proxy_pass.
Can someone explain why the first case isn't working? What is the proper solution? Note that I need to keep the variable in proxy_pass so it uses the resolver.
Side note: I use a trailing slash in my location block because otherwise I get a 404 when I use this URL:
domain.com/nzbget/api/api:password/xmlrpc
This is why I use /nzbget/api/ instead of /nzbget/api.
EDIT 1
I played around with this some more, and I found that this also doesn't work:
location /nzbget/api/ {
set $upstream_nzbget http://nzbget:6789/;
proxy_pass $upstream_nzbget;
}
This one is really strange. It's the same string, the only difference is using a variable vs a string literal. I'm not doing any string concatenation here. I'm not sure why there's a behavioral difference.
EDIT 2
This SO question might be the same issue, but it has no helpful answers.

Answer 1

You need to pass $request_uri when using it with variable. It's there in the docs somewhere.
It'll be something like:
location /nzbget/api/ {
set $upstream_nzbget http://nzbget:6789$request_uri;
proxy_pass $upstream_nzbget;
}