IE 9 Javascript error c00c023f

Question

I came across this error only on IE9:

SCRIPT575: Could not complete the operation due to error c00c023f.

The error happened on this line: if ((a.responseXML) && (a.readyState==4)) {

I cant figure it out why this happened, and it seems to work very well in other browsers.

and this is my javascript code:

var a = new XMLHttpRequest();
a.open("GET",'/cust/ajax/getresult.php?qk=nnf87&arg1='+pzid,true);
a.onreadystatechange = function () {
    if ((a.responseXML) && (a.readyState==4)) {
        var N = a.responseXML.getElementsByTagName('result')
        sequence = N[0].firstChild.data;
        var SEQ = sequence.split(",");
        var num = SEQ.length;
                    var sum = 0;
                    for(var n=0;n<num;n++){sum = sum + (SEQ[n]*1);}
        //document.getElementById("the_number_of").innerHTML = sum;
        var date = new Date();
        date.setTime(date.getTime()+(2*60*60*1000));
        document.cookie='cpa_num='+sum+'; expires= '+date.toGMTString()+'; path=/';
    }

}

Answer 1

Switch the

if ((a.responseXML) && (a.readyState==4))

to

if ((a.readyState==4) && (a.responseXML))

As the order matters. it seems that on IE9 if the state is not 4, the responseXML and reponseText yield this error if being accessed (I have no clue why...)