aDiv[j].style.display="none";代码： Uncaught TypeError: Cannot read property 'style' of undefined

window.onload=function()
{
        var oTab=document.getElementById("month_tit");
        var oTac=document.getElementById("month_con");
        var aH3=oTab.getElementsByTagName("li");
        var aDiv=oTac.getElementsByTagName("div");
        for(var i=0;i<aH3.length;i++)
        {
                aH3[i].id=i;
                aH3[i].onmouseover=function()
                {
                        for (var j=0;j<aH3.length;j++)
                        {
                                aH3[j].className="";
                                aDiv[j].style.display="none";
                        }
                        this.className="select";
                        aDiv[this.id].style.display="block";
                }
        }
}