Cannot set property 'onmousedown' of undefined QQ面板拖拽请问什么原因引起的

Question

function getByClass(clsName,parent){

var oParent=parent?document.getElementById(parent):document,

eles=[],

elements=oParent.getElementsByTagName('*');

for(var i=0,l=elements.length;i<l;i++){

if(elements[i].className==clsName){

eles.push(elements[i]);

}

}

return eles;

}

window.onload=drag;

function drag(){

var oTitle=getByClass('login_logo_webqq','loginPanel')[0];

oTitle.onmousedown=fnDown;

}

function fnDown(){

document.onmousemove=function(event){

event=event||window.event;

document.title=event.clientX+","+event.clientY;

}

}