javascript to change element class

This should be simple. If a form field doesn't validate, I want to turn the label for that field red and give a pop-up. To color the text, I've used the SetAttribute method to set the class to one declared as having red text in my CSS.

Works like a charm in Firefox (of course), but not in IE.

Can anyone point me in the direction of WHY it doesn't work in IE?

The JavaScript
function checkZip(fld)
{
var val=fld.value;
var Dgts=val.match(/d/g);
if(Dgts==null || (Dgts.length!=5 && Dgts.length!=9 ))
{
alert("The zip code you entered is invalid.");
fld.setAttribute('class','textRed');
}
else
{
fld.setAttribute('class','textBlack');
}
}

The CSS:
.textRed
{
color:red;
}

Any pointers would be a big help. This crap is making me nuts.

Thanks,
Ursus