Dynamically Changing Text Box Value From A Select Box

I have a drop down box on a site, and when an option is selected, I need the value of a text box to change.

My form has the following in it:

<select name="event">
<option value="" onclick="updateprice(0)">Please Select..</option>
<option value="3" onclick="updateprice(25)">Monday 16th February 2004 (£25 per ticket)</option>
<option value="2" onclick="updateprice(20)">Saturday 28th February 2004 (£20 per ticket)</option>
</select>

and I have this javascript function:

function updateprice(theamount) {
document.nonmember.adminprice.value = theamount
}
the form is called 'nonmember' and the text box i need to change is called 'adminprice'.

Everything works fine in mozilla, but in IE nothing happens when I select an item from the drop down list (not even a javascript error)