Can't use with Surface Pen

[matthewfcarlson]

Awesome project. Tried to use my surface pen and it didn't like it. My guess it the way that input is handled.
Uncaught TypeError: Cannot read property 'clientX' of undefined
at alert_coords (draw:100)
at mouseUp (draw:121)
at SVGSVGElement.ontouchend (draw:87)
alert_coords @ draw:100
mouseUp @ draw:121
ontouchend @ draw:87

https://imgur.com/a/Yh9QbIT