What is a valid MediaStreamTrack

[youennf]

The spec says:

If init.[track](https://w3c.github.io/mediacapture-transform/#dom-mediastreamtrackprocessorinit-track) is not a valid [MediaStreamTrack](https://www.w3.org/TR/mediacapture-streams/#dom-mediastreamtrack), throw a [TypeError](https://webidl.spec.whatwg.org/#exceptiondef-typeerror).

But a valid MediaStreamTrack is not defined.
Is it that this should be a video live track?

[youennf]

Chromium seems to throw on ended tracks.
Safari is also throwing on audio tracks.