typeahead.js: problem with source async

$('#input-rms').typeahead({
  hint: true,
  highlight: true,
  minLength: 2,
  classNames: {
    menu: 'tt-dropdown-menu'
  }
},
{
  name: 'rms',
  limit: 10,
  source: function (q, sync, async) {
    $.ajax('/api/getrms?name='+q, // it will return a list of data of length 7
           {
            success: function(data, status){  async(data); }
           }
          );
  }
});

Problem: The ajax url will return a list of data of length 7, however, the suggestions only render the first 3 results.

Can anyone gives a example about how to use the async in source.