error thrown in createOffer for a rejected datachannel

[fippo]

    describe('rejects a legacy datachannel offer', () => {
...
      it.only('in setRemoteDescription with a local track', () => {
        // test that we don't forget about the track.
        return navigator.mediaDevices.getUserMedia({audio: true})
          .then((stream) => {
            pc.addTrack(stream.getTracks()[0], stream);
            return pc.setRemoteDescription({type: 'offer', sdp: sdp});
          })
          .then(() => pc.createAnswer())
          .then((answer) => pc.setLocalDescription(answer))
          .then(() => pc.createOffer())
          .then((offer) => {
            const sections = SDPUtils.getMediaSections(offer.sdp);
            expect(sections).to.have.length(2);
            expect(SDPUtils.isRejected(sections[0])).to.equal(true);
            expect(SDPUtils.kind(sections[1])).to.equal('audio');
          });
      });

This does weird stuff in createOffer, creating an ice gatherer for the rejected m-line and then failing.