为什么会报错？是这样获取的啊

Uncaught TypeError: Cannot read property 'getElementsByTagName' of null

    at window.onload (11.html:33)

<script>

window.onload = function(){

var box = document.getElementById('content');

var imgs = box.getElementsByTagName('img');

var imgWidth = imgs[0].offsetWidth;

var exp = 180;

var boxWidth = imgWidth + (imgs.length - 1)*exp;

box.style.width = boxWidth + 'px';

for(var i=1,len=imgs.length;i<len;i++){

imgs[i].style.left=imgWidth+exp*(i-1)+'px';

}

};

</script>