SetAttribute Erroe

[paulmac106]

Hi,

I'm trying to fix a page that another person created, but I keep
getting an error.

Functionality: If a checkbox is checked I want a field to be not
required. Here is my function:

function ClaimantZipIsUnknown() {
var ClaimantZipUnknown = 'ctl00$ContentPlaceHolder1$examineezip';
if (GetElement('<% WriteClaimantZip() %>').checked) {
ClaimantZipUnknown.removeAttribute('required');
} else {
ClaimantZipUnknown.setAttribute('required','true');
}
}

But I get an error message saying that I can't set the property for
that object(although it is done somewhere else on the page.

Any ideas what I'm doing wrong?

thanks for your help