No, the real proof of a successful Live Lounge cover is that it sounds like something the covering artist could have written and recorded themselves, no matter how unlikely the source material.
And that's exactly what Florence "+" The Machine have done today, with their cover of Jack U and Justin Bieber's Where Are U Now? Admittedly, Florence only has one musical gear ("foghorn in a hurricane") so it would be hard for her to make the cover sound like anything or anyone else - but when she delivers performances like these, who cares?
The cover comes at the end of this six-song special. Skip to 23 minutes into the embedded player below.
PS: That player expires on 28 October, so apologies if you're visiting the page after that date.