b[d].push is not a function

[netsi1964]

On this Pen on Codepen CSS transform-origin visuzlized In Safari Version 9.0.2 (11601.3.9) I get this error:

TypeError: b[d].push is not a function. (In 'b[d].push(e)', 'b[d].push' is undefined)
cweb-animations.min.js:345
convertEffectInputweb-animations.min.js:376
KeyframeEffectweb-animations.min.js:491
animateweb-animations.min.js:628

[archanox]

I've been having this issue for a while too.
I just tried updating today to web-animations-next.dev.js to see if it has been resolved in a prerelease.
Now I'm getting the error

TypeError: propertySpecificKeyframeGroups[member].push is not a function. (In 'propertySpecificKeyframeGroups[member].push(propertySpecificKeyframe)', 'propertySpecificKeyframeGroups[member].push' is undefined)