Accessing a label via javascript

In the sql reporting report viewer control, when a value can be set to null it has a checkbox called null.

So the "NULL" text is contained within a label. I need to change this text to "ALL" because users just don't understand the word "NULL"

 I I wrote this code:

labels = document.getelementsByTagName('label');

for( var i = 0; i < labels.length; i++ ) {

if ( labels[i].innerText == 'NULL' )

    labels[i].innerText = 'ALL';

}

 

But I can't seem to get it to work. Any ideas?