replaceTrack only works once

I'm looking at implementing a "switch between front and rear camera" feature.

When user taps a button to switch the camera:

* I run `navigator.mediaDevices.getUserMedia` with appropriate constraint
* when getUserMedia returns a stream I run `replaceTrack` like so:

```
navigator.mediaDevices.getUserMedia({
video: { facingMode: facingMode },
audio: true
}).then(function(stream) {
if (p && localStream) {
p.replaceTrack(localStream.getVideoTracks()[0], stream.getVideoTracks()[0], localStream);
console.log("Done");
}

localStream = stream;
}).catch(() => {})
```

In the example, `p` is a Peer instance, and `localStream` is a local variable that keeps track of the current local stream. The above works, but only once. When I try to switch camera the second time, the replaceTrack doesn't return (the `console.log`statement doesn't run), and there are no errors logged in console.

Am I using it wrong? Is there an example of an idiomatic use of `replaceTrack`?