Unable to set media.sense_mode for IMZ-320

I used the following command to set the media settings of my IMZ-320 printer, but when I execute ! U1 getvar "media.sense_mode", it always returns "bar".

! U1 SPEED 3

! U1 setvar "print.tone" "100"

! U1 setvar "media.type" "label"

! U1 setvar "media.sense_mode" "gap"

What am I doing wrong?

Thanks,

Noob