GetElementByID Not Working With Array

Why does'nt the arrayvalue work?
I'm sure its foo. Is it some string conversionproblem. I've tried a lot but nothing works.

Code:
function handleResponse() {
if(http.readyState == 4){
var response = http.responseText;
var update = new Array();

if(response.indexOf('|' != -1)) {
update = response.split('|');
document.getElementById(update[0]).innerHTML = update[1]; //Dont work.
//document.getElementById('foo').innerHTML = update[1]; //Works!
}
}
}