jquery.min.js:4 Uncaught TypeError: Cannot read property 'temperature' of undefined

Есть javascript код, который выдаёт такую ошибку:

 var period = 1;
var currentTemperature = <?php echo $currentThemperature; ?>;
var maximum = <?php echo $info['maximum'] ?>;
var minimum = <?php echo $info['minimum'] ?>;
var average = <?php echo $info['average'] ?>;
function checkPeriod(){
    period = document.getElementById('period').value;
}
var json = (function () {
    var json = null;
    $.ajax({
        async: false,
        global: false,
        url: '/test/<?php echo $info['hub_id'] ?>/<?php echo $info['position'] ?>',
        type: 'POST',
        dataType: "json",
        data: {period: period},
        success: function (data) {
            json = data;
            currentTemperature = data[data.length-1].temperature;
        }
    });

Что означает эта ошибка?