TypeError when using attribute starting with on

[jacekkarczmarczyk]

Version

3.0.0-alpha.12

Reproduction link

https://codepen.io/jkarczm/pen/YzyPwYJ?editors=1010

Steps to reproduce

1. Open codepen - nothing renders, there's an error in console
2. Change "onwards" to something else that doesn't start with "on" - "OK" is rendered

What is expected?

should render "OK" with "onwards" prop

What is actually happening?

TypeError

In case if this is expected and attribute names starting with "on" are reserved it should be clearly stated in docs

Edit: probably this line https://github.com/vuejs/vue-next/blob/master/packages/runtime-dom/src/patchProp.ts#L29