Add 20th GRE tunnel "tun 20" on WR41v2

Dear All,

I want to add
tun 20
but I have get ERROR.

When type
tun ?
I get
ss######>tun ?
Format "TUNNEL <0…19>

How to configure 20th GRE tunnel?
Is it possible to add more tunnels and have
tun 20
tun 21

Thanks in advance.

Details about the router,

Model: Digi TransPort WR41v2
Part Number: WR41-U800-DV1-XX

Firmware Version: 5180 $ (Jan 10 2013 09:45:15)
SBIOS Version: 6.97u
Build Version: MV
HW Version: 3203a