番茄钟计时器:变量值变为'NaN'

我正在尝试构建像http://codepen.io/GeoffStorbeck/full/RPbGxZ/这样的番茄钟.秒值随机变为NaN,然后​​在"break"开始后恢复正常.

$('#circle a').click(function() {
  var timer = $('.time > span').html();
  timer = timer.split(':'); 
  var minutes = timer[0];    //Value of minutes
  var seconds = timer[1];    //Value of seconds

  var settimer = setInterval(function() {
    seconds -= 1;
    console.log(seconds);
    if (seconds < 0 && minutes != 0) {
      minutes -= 1;
      minutes = String(minutes);
      seconds = 59;
    } else if (seconds < 10 && seconds.length != 2)
      seconds = '0' + seconds; 
    if (minutes < 10 && minutes.length < 2) 
      minutes = '0' + minutes;

    $('.time > span').html(minutes + ':' + seconds);

    //Start break when session is completed
    if (minutes == 0 && seconds == 0) {
      $('.upper').find('h1').text('BREAK');
      var time = $('#break').find('span').text();
      $('.time > span').html('0' + time + ':00');
      $('#circle a').trigger("click");  //Start timer for break
    }
  }, 1000);
});

这是codepen的链接 http://codepen.io/ibrahimjarif/pen/wMKJWN

我如何解决NaN问题？有没有更好的方法来实现这个？