Fortigate 100E cannot ping wan2

[fang]

Hello i've configured my wan2 in this way:

edit "wan2"
        set vdom "root"
        set ip 20.20.z.y 255.255.224.0
        set allowaccess ping https ssh fgfm
        set type physical
        set alias "MY WAN2"
        set role wan
        set snmp-index 4
    next

i also have a pc in the same subnet with an IP address like 20.20.x.y

The problem is that i am not able to do ping 20.20.z.y from the machine 20.20.x.y. I obtain:

Destination host unreachable

any idea?

Thanks