Form With DropDown And Input Field - Function Error

New to working with javascript and I've come up with a function. I have a form with a dropdown and input field:
<form>
<select name="warranty" id="warranty" onChange="doadate()">
<option value="Select">Select</option>
<option value="In Warranty">In Warranty</option>
<option value="Out of Warranty">Out of Warranty</option>
<option value="DOA" selected>DOA</option>
</select>
<input name="doadate" type="text" id="doadate" value="<? echo $doadate; ?>"
size="20" readonly>
</form>

I need the input field to be disabled unless DOA is selected in the select form element. I've started on page load with document.form1.doadate.disabled=true. Then I've put an onChange handler in the select to call the function below:
<script type="text/javascript">
function doadate(){
val = document.form1.warranty.value;
if(val=='DOA'){
document.form1.doadate.disabled=false;
} else{
document.form1.doadate.disabled=true;
}}
</script>
But it's not working, it stays disabled after load even when the DOA is selected in the drop down form field.