Select2 - Create new option

I'm using the "createTag" option of the Select2 plugin to allow the user to type in the search and also allow to include what was typed as a selected option.

But it only works when options already exist in select. If it is empty (returning values by Ajax) does not allow creating the new option.

$('.nova_despesa_mensal.pago_a').select2({language:"pt-BR",
                width: '100%',
                ajax: {
                    url: '/financeiro/transacoes/contatos_financeiro',
                    dataType: 'json'
                },
                tags: true,
                // add "(new tag)" for new tags
                createTag: function (params) {
                    var term = $.trim(params.term);

                    if (term === '') {
                        return null;
                    }

                    return {
                        id: term,
                        text: '+ Adicionar ' + term,
                        isNew : true
                    };
                }
            });