'ip link set mtu 1280 dev utun1' error,how to fix it?

[luckypoem]

hi.

yudeMacBook-Air:~ brite$ sudo simple-vpn client simple-vpn-client.cfg
Password:

Client IP is 10.137.248.2
Running: 'ip link set dev utun1 up'
Executing: /usr/bin/sudo /sbin/ifconfig utun1 up

Running: 'ip link set mtu 1280 dev utun1'
Executing: /usr/bin/sudo /sbin/ifconfig mtu mtu 1280
ifconfig: interface mtu does not exist
Usage: ip link show [ DEVICE ]
ip link set dev DEVICE
[ { up | down } ]
[ address { LLADDR | factory | random } ]
[ mtu MTU ]
Failed to run ip link set mtu 1280 dev utun1 - exit status 255panic: exit status 255

goroutine 5 [running]:
main.(*clientCmd).Execute.func2(0xc420136080, 0x4, 0x4, 0x4, 0xc420156158)
/Users/brite/go1.10.2/go/gopath/src/github.com/skx/simple-vpn/cmd_client.go:251 +0x83c
github.com/skx/simple-vpn/shared.(*Socket).Serve.func1(0xc4200aa7e0, 0xc420048100)
/Users/brite/go1.10.2/go/gopath/src/github.com/skx/simple-vpn/shared/socketHandler.go:368 +0x51d
created by github.com/skx/simple-vpn/shared.(*Socket).Serve
/Users/brite/go1.10.2/go/gopath/src/github.com/skx/simple-vpn/shared/socketHandler.go:264 +0x118
yudeMacBook-Air:~ brite$