mpris:length returns an s when i think it should return whatever x is #341

The player I am using is [io].

playerctl metadata --format "Total length: {{ duration(mpris:length) }}"

This command (mentioned in the readme) returns an error on my machine
Specifically, this one :-

Could not execute command: function duration can only be called on track position values

i'm using it to rice my waybar to make a player widget from scratch