javascript - Cannot set property 'innerHTML' of null?

Why do I get an error or " Uncaught type error: Cannot set property 'innerHTML' of null?".
This is my code

function ajax()
 {
var pathxml;
var answer;
if (window.XMLHttpRequest) {
  pathxml = new XMLHttpRequest();

}
else
{
    pathxml = new ActiveXObject("Microsoft.XMLHTTP");
}

pathxml.onreadystatechange= function()
{
    if (pathxml.readyState==4 && pathxml.status==200) 
    {

         answer = pathxml.responseText;

    document.getElementById("ans").innerHTML = answer;

    }
 }
 pathxml.open("GET","books/ax",true);
 pathxml.send();
  }