Random Number Generator Pop Up Error

Hi,

I have a html form on a web page that I am using a hidden field that has a random number inserted by javascript.

[HTML]
<script>
function genRandomNumber() {
document.form.hidden.value = Math.floor(Math.random()*100);
alert(document.form.hidden.value);
}
</script>[/HTML]
I call this script with onload="MM_callJS('window.onload = genRandomNumber();

It works fine and the random number is inserted in the hidden field...but when the page loads a IE pop up error box shows up with the random number and on OK box.

Is there a way to keep this error box from showing up? I am using IE 7.

Thanks for any input

What does the error message say? Does it only occur in IE or on other browsers too?