proxy_ssl_certificate not working as expected (no replies)

Hello,

I'm using nginx as a proxy to a backend server.
The backend server is also using nginx and enforcing client certificate authentication using the ssl_client_certificate and ssl_verify_client directives.

In my nginx server I set the following:

location /proxy {
proxy_pass https://www.backend.com;

proxy_set_header X-Forwarded-Host $host;
proxy_set_header X-Forwarded-Server $host;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

proxy_ssl_certificate /etc/nginx/cert/client.crt;
proxy_ssl_certificate_key /etc/nginx/cert/client.key;
}

according to http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_ssl_certificate.

However, the backend is still responding with a 400 reponse code "No required SSL certificate was sent".

Note that when issuing requests to the backend server using wget with the client certificate, I get a valid 200 OK response.

What am I missing in my nginx configuration?