zoompic插件中通过对图片双击或双指放大，在控制台报错

[harmsworth]

iSlider.plugin.zoompic.js?9af6:285 Uncaught TypeError: Cannot read property 'pageX' of undefined
at handleDoubleTap (iSlider.plugin.zoompic.js?9af6:285)
at iSlider.endHandler (iSlider.plugin.zoompic.js?9af6:314)
at iSliderPrototype.handleEvent (iSlider.js?c3de:1183)

这一段代码
/**
* Double tao handle
* @param {object} evt
*/
function handleDoubleTap(evt) {
var zoomFactor = zoomFactor || 2;
var node = zoomNode;
var pos = getPosition(node);
currentScale = currentScale == 1 ? zoomFactor : 1;
node.style.webkitTransform = generateTranslate(0, 0, 0, currentScale);
if (currentScale != 1) node.style.webkitTransformOrigin = generateTransformOrigin(evt.touches[0].pageX - pos.left, evt.touches[0].pageY - pos.top);
}

pageX报错