Javascript to get selectedIndex on my dropdown list

I amt trying to get the selectedIndex on my dropdown list, but I am getting ‘object required’ error. Please review my code and suggest correction:
<script type=“text/javascript”>
function EnablePanel() {
index = this.document.getElementById(‘adv_client’).selectedIndex;
if (index > 0) this.document.getElementById(‘panel3’).Enabled = true;
}
</script>

Thank You.

try

 
index = document.getElementById('adv_client').selectedIndex;

and make sure the id of the <select> is ‘adv_client’

I still get the error: Microsoft JScript runtime error: Object required


<script type="text/javascript">
    function EnablePanel3() {
        index = document.getElementById('adv_client').selectedIndex;
        if (index > 0) document.getElementById('panel3').Enabled = true;
    }
</script>



 <asp:Panel ID="panel2" runat="server" GroupingText="CLIENT" EnableViewState="true" >
 <tr>
                            <td class="cls">CLIENT</td><td><asp:DropDownList ID="adv_client" runat="server" OnInit="adv_client_Init" ></asp:DropDownList></td>
                        </tr>

   </asp:Panel>
                    </table> 
                    <asp:Panel ID="panel3" runat="server" GroupingText="Doctors" EnableViewState="true" >
                    <table> 
                        <tr>
                            <td class="cls">Facility1</td><td><asp:DropDownList ID="adv_ord_ddl" runat="server" 
                                onselectedindexchanged="adv_ord_ddl_SelectedIndexChanged" 
                                 oninit="adv_ord_ddl_Init" ></asp:DropDownList>
                            </td>
                        </tr>
                        <tr>
                            <td class="cls">Facility2</td><td><asp:DropDownList ID="adv_treat_ddl" runat="server" OnSelectedIndexChanged="adv_treat_ddl_SelectedIndexChanged"
                             OnInit="adv_treat_ddl_Init"></asp:DropDownList></td>
                        </tr>
                        </asp:Panel>
                    </table>      

A test page that demonstrates the problem, will enable us to apply some debugging techniques to find and fix the problem.

If you cannot provide a test page, posting the rendered HTML code will be more helpful than the ASP source code.