OnChange not working in IE 6

I have the following code setup on a webform:

 <tr>
            <td>
                <label for="EmailAddress">
                    Email Address</label><br />
                <input type="text" class="cat_textbox" id="EmailAddress" maxlength="255" name="EmailAddress"
                    onchange="document.getElementById('Username').value=this.value;" />
            </td>
        </tr>
        <tr class="hidden">
            <td>
                <label for="Username">
                    Username</label><br />
                <input type="text" class="cat_textbox" id="Username" maxlength="255" name="Username" />
            </td>
        </tr>

The onchange event is not working in IE 6.  Any ideas why?