ER-X SFP v1.8.5 can't enable "HW NAT" offload

Hi,

ER-X SFP upgrade to v1.8.5:

Linux ubnt 3.10.14-UBNT #1 SMP Sat May 28 09:48:10 PDT 2016 mips
Welcome to EdgeOS

ubnt@ubnt:~$ show version
Version: v1.8.5
Build ID: 4884695
Build on: 06/08/16 10:57
Copyright: 2012-2016 Ubiquiti Networks, Inc.
HW model: EdgeRouter X SFP 6-Port
Uptime: 06:20:25 up 14 min, 1 user, load average: 1.22, 1.15, 0.74

and then enable HW NAT offload:

ubnt@ubnt:~$ set system offload hwnat enable
Invalid command

but it show that the command is invalid. Any thing wrong?

Thanks!