Safari 11 mediaElement['srcObject'] = mediaStream throws Type Error

Question

I am trying to implement WebRTC using this library (RTCMulticonnection) and am encountering a Type Error when trying to set the 'srcObject' on the video mediaElement in Safari Tech Preview 11.

Relevant code:

if (isSafari) {

    //I've tried the 4 ways below to set the srcObject but all cause a Type Error

      // mediaElement['srcObject'] = window.URL.createObjectURL(stream); throws type error
      // mediaElement['srcObject'] = stream; throws type error
      // mediaElement['src'] = window.URL.createObjectURL(stream);
      // mediaElement.srcObject = stream; // throws type error
}

When I print mediaElement, I get: <video></video>

When I print stream, I get a MediaStream object.

MediaStream {streamid: "722cfcc7-0687-4265-b266-e81b2abf8398", idInstance: "{\"audio\":{\"mandatory\":{},\"optional\":[{\"bandwidth\":…\":[{\"bandwidth\":4194304},{\"facingMode\":\"user\"}]}}", isVideo: 1, isAudio: false, type: "local", …}

Error

TypeError: Type error
[Error] No message (2)
TypeError: Type error — RTCMultiConnection.js:4089
Object audio: {mandatory: {}, optional: [{bandwidth: 1048576}]}
    video: {mandatory: {}, optional: [{bandwidth: 4194304}, {facingMode: "user"}]}

Object Prototype
    onMediaError (RTCMultiConnection.js:5215)
    onLocalMediaError (RTCMultiConnection.js:806)
    onLocalMediaError (RTCMultiConnection.js:4995)
    (anonymous function) (RTCMultiConnection.js:3365)
    promiseReactionJob

The Type Error is caught in this then/catch:

navigator.mediaDevices.getUserMedia(options.localMediaConstraints).then(function(stream) {
      stream.streamid = stream.streamid || stream.id || getRandomString();
      stream.idInstance = idInstance;
      streaming(stream);
  }).catch(function(error) {
      options.onLocalMediaError(error, options.localMediaConstraints             });