GLib-GObject-CRITICAL when using firefox

When using the following playerctl command:

playerctl metadata --format '{{ playerName }}: {{ artist }} - {{ title }} {{ duration(position) }}|{{ duration(mpris:length) }}' --follow

and after that, starting playback on Firefox (for example, on this page: https://thegarrys.bandcamp.com/track/cakewalk), I get the following output:

(playerctl:2400): GLib-GObject-CRITICAL **: 09:14:55.667: g_value_get_long: assertion 'G_VALUE_HOLDS_LONG (value)' failed

Not sure if this qualifies as a bug, is the assertion failure expected here?