Can't get chai.spy.on to work

Please don't suggest to use Sinon. I want to get chai-spies specifically chai.spy.on working with your help. Basically, I have this spec. Inside my initialize method in PatientController, I call this.initializePatientEvents();

beforeEach(function() {
  this.patientController = new PatientController({model: new Backbone.Model(PatientModel)});
});

it('executes this.initializePatientEvents', function () {
  let spy = chai.spy.on(this.patientController, 'initializePatientEvents');
  expect(spy).to.have.been.called();
});

However, the test is failing with this error

AssertionError: expected { Spy } to have been called
at Context.<anonymous>

I spent almost 3 hours now with no luck! :(