Issue forwarding https to private server

Hi,

i have a Fortigate 70D with 5.4. I need to forward https with usual port 443 from external public IP to a private server.

So, i have created all the keys and certificates for this purpose.

Created a VIP and then a policy to forward the port.

But this is what happens:

In private LAN, when i https to this server, it pops up its certificate.

In Internet, when i https to this server, first pops up the Fortinet self-signed certificate and then the private server web certificate.

Is there something that i do not know?

Many Thanks,

            Antonio