How to mock a Firebase set() with Sinon?

Question

I am trying to test this Firebase access code

let writeHomePromise = admin
  .database()
  .ref(
    "sm_matches/" +
    match.date +
    "/" +
    match.homeTeam.name +
    "/homeNGSPlayers"
  )
  .set(match.homePlayers);

using Sinon.

I create a match with

    const match = {
        'date' : '2902199',
        'homeTeam' : '{"name" : "My_team"}',
        'homePlayers'  : ["Nomran Mailer", "Peter Bonetti"],
        'awayPlayers': ["Ernst Blofeld", "Postman Pat"],
      };

And I am trying, unsuccessfully, to mock that code with

    let refStub = sinon.stub();
    let setStub = sinon.stub();
    refStub.withArgs('sm_matches/2902199/My_team/homeNGSPlayers').returns(
        {push: () => ({key: 'fakeKey', set: setStub})}
        );

which gives me TypeError: Cannot read property 'set' of undefined.

This is my first foray into Sinon, and even something this simple is giving me problems.

What am I doing wrongly?