Is it possible to assign a zone to the ssl.root interface?

I am new to Fortigate and started using FortiOS 5.6.3.

I finally setup SSL-VPN and I am able to assign the ssl.root interface to firewall policies and they work quiet fine. However I wounder if it is possible to assign the interface to a zone. It somehow does not appear in the zone assignment screen.

I allready made sure that the interface is not used in any policy but without any luck. So I would be happy if someone can just confirm that it should work... any hint what I may do wrong might be helpfull aswell.

Best regards and thank you in advance.