Haproxy: how to configure backend to talk with a https server?

Question

We have in our LAN a nginx server configured in https.

In front, we have Haproxy with SSL and we need to establish a connection SSL from backend to nginx server. Is it possible to do this?

Client -> Haproxy:SSL -> SSL:Nginx

backend pkiservices
    mode http
    option httplog
    option forwardfor
    server pki pkiservices.example.com:443

With this configuration, Nginx return an erreur 400 (The plain HTTP request was sent to https port).

Thanks in advance