selectize.js - Unable to search for integer values

I just integrated the selectize.js plugin, using the ajax results. This is my init config:

$('#user_select').selectize({
    valueField: 'id',
    labelField: 'fullname',
    searchField: 'fullname',
    delimiter: ',',
    plugins: ['remove_button'],
    render: {
        option: function(item, escape) {
            return '<div>' + escape( item.firstname + ' ' + item.lastname + ' (' + item.id + ')' ) + '</span></div>';
        }
    },
    load: function(query, callback) {
        if (!query.length) return callback();
        $.ajax({
            url: '...',
            type: 'GET',
            dataType: 'json',
            data: {
                search: query
            },
            error: function() {
                callback();
            },
            success: function(res) {
                callback( res.data );
            }
        });
    }
});

Searching for complete strings or strings in combination with integers is no problem. Also the field value is correctly set.

But if I search only for integer values, the ajax gets started, the success action is being loaded correctly and the response data is being delivered to the callback function. Thereafter: no reaction at all. The ajax response itself is completely correct.

I already checked the documentation a few times, but I'm not able to find something. Is this maybe a bug? Do I miss an option somewhere?