i have two spans on my page with class='hidden' and then some javascript to remove the class when a condition is met, its working fine in ie 9/10 and firefox but its not working in chrome when I run the function in the chrome JS console I get the message TypeError: Cannot read property 'attributes' of null

Anybody know whats going on?

<script type='text/javascript' >
function showhidden()   {

        var att =document.getElementById('hiddentextbox');
        att.attributes[0].value='';
        att =document.getElementById('hiddentextbox1');
        att.attributes[0].value='';
}</script>

Thanks