'theForm.__EVENTTARGET' is null or not an object.

Ever had the following problem: 'theForm.__EVENTTARGET' is null or not an object? I will reproduce this problem in this site and suggest a possible solution for this problem.

I came across this when converting a ASP.Net 1.1 website to .Net 3.5. It has to do with a change made to optimize the javascript code generated by .Net concerning the postback. In 1.1. the code is executed every time, but in 2.0 some initialization is done at document load time.

.Net 1.1 style:
function __doPostBack(eventTarget, eventArgument) {
var theform;
if (window.navigator.appName.toLowerCase().indexOf("microsoft") > -1) {
	theform = document.form1;
else {
	theform = document.forms["form1"];
theform.__EVENTTARGET.value = eventTarget.split("$").join(":");
theform.__EVENTARGUMENT.value = eventArgument;
.Net 2.0 style
var theForm = document.forms['form1'];
if (!theForm) {
theForm = document.form1;
function __doPostBack(eventTarget, eventArgument) {
if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
    theForm.__EVENTTARGET.value = eventTarget;
    theForm.__EVENTARGUMENT.value = eventArgument;

It occurs when you have javascript on the page that changes the HTML DOM in a particular way (using: document.body.innerHTML = ...). You should change your javascript, or if you really need it, use this solution. Look at my javascript file to identify the problem and to see a part of the solution. Ok, let's go and reproduce the problem.

Selecting a deferent value will cause a postback (and the javascript error):
You have selected:

Check the checkbox and click Submit to enable the fix.

   The Fix was not applied.

Now try selecting a diferent value again.

You can download the sources of this solution here.