custom format of backlight module not working

[marvin-steinke]

"backlight": {
    "device": "intel_backlight",
    "format": "L: {percent}%",
    "format-off": "L: off",
    "states": {
        "on": 1,
        "off": 0
    },
},

When in state "off", "L: 0%" instead of "L: off" is displayed. Styling the state, e.g. #backlight.off {color: #a89984;}, is working though.