Chaining Ajax calls fail after upgrading jQuery to 3.x

I'm working on a very old project with the following frameworks for unit testing: QUnit 1.14.0, Sinon 1.17.2, jQuery 1.12.4

This unit test works fine with jQuery 1.x and 2.x. But after upgrading to 3.x, I got error requests[1] is undefined, which means the second AJAX call is not caught. I read the 3.0 doc and it seems 3.0 has a new version of Deferred which was changed to behave like the native Promise object. But I'm not sure how that could screw up the tests.

function doSomething(successCallback, failureCallback) {
    var promise = firstCall()
           .then(secondCall);

    if (successCallback) {
        promise.done(successCallback);
    }
        
    if (failureCallback) {
        promise.fail(failureCallback);
    }
    return promise;
}

function firstCall() {
    return $.ajax({
           url: 'No1.aspx',
           type: 'HEAD'
        });
}
function secondCall() {
    return $.ajax({
           url: 'No2.aspx',
           type: 'GET'
        });
}

QUnit.test('correct sequence of AJAX calls', function (assert) {
    var server = this.sandbox.useFakeServer();

    doSoemthing(this.successHandler, this.failureHandler);

    // Simulate successful responses to the two expected AJAX requests
    server.requests[0].respond('success');
    server.requests[1].respond('success');

    assert.equal( 1, "1", "dummy" );
});