nginx proxy to external proxy

Hi! I need to configure an nginx proxy that directs requests to another proxy and I'm having some trouble. Here's the context: We have machine A from where the request comes from, machine B where the nginx proxy will be and machine C where an external proxy is (squid). Only C has access to the internet and A has no connectivity with C. Therefore I need to go from A to B to C to the required website.

Here's what my conf looks like (the relevant part):

server {
listen Bport;
location / {
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header Host $http_host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_pass http://Cip:Cport;
}
}

But whenever I do something like:

curl -v -x http://Bip:Bport https://google.com

I get:

About to connect() to proxy Bip port Bport (#0)
* Trying Bip... connected
* Connected to Bip (Bip) port Bport (#0)
* Establish HTTP proxy tunnel to google.com:443
> CONNECT google.com:443 HTTP/1.1
> Host: google.com:443
> User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.19.1 Basic ECC zlib/1.2.3 libidn/1.18 libssh2/1.4.2
> Proxy-Connection: Keep-Alive
>
< HTTP/1.1 400 Bad Request
< Server: nginx/1.10.1
< Date: Tue, 11 Aug 2020 10:03:27 GMT
< Content-Type: text/html
< Content-Length: 173
< Connection: close
<
* Received HTTP code 400 from proxy after CONNECT
* Closing connection #0
curl: (56) Received HTTP code 400 from proxy after CONNECT

Whereas if I use the external proxy directly (starting from B):

curl -v -x http://Cip:Cport https://google.com

The request goes through and everything works OK. Can someone provide some insight on this? Thanks in advance.