SinonJS calledOnce or callCount issue for a inner object function

Running tests with SinonJS 1.10.x (sorry, can't update the version) I'm facing the following issue

Error: PhantomJS:expected 0 to equal 1near

What's wrong with the test?

    var spyRequester = sinon.spy(util._requesterHandler, '_doCreate');

    var result1 = util._requesterHandler.createRequesterIfNotExists(url);
    var result2 = util._requesterHandler.createRequesterIfNotExists(url);

    assert.equal(spyRequester.callCount, 1); // assertion error

And requestHandler API is

    requesterHandler = {
        createRequesterSubscribers: function createRequesterSubscribers(url) {...},
        createRequesterIfNotExists: function createRequesterIfNotExists(url, options) {...}
        _doCreate: doCreate
    }

the piece of code which calls doCreate inside createRequesterIfNotExists

    createRequesterIfNotExists: function createRequesterIfNotExists(url, options) {

      var shouldCreate = typeof requesterMap[url] === 'undefined';
      var optionsArray = (requesterMap[url] || {}).optionsArray || [];
      optionsArray.push(options);

      if (shouldCreate) {
        requesterMap[url] = doCreate(url, optionsArray); // call doCreate
      }