AsyncPostBackTrigger in UpdatePanel

Could not find an event named 'OnSelectedIndexChanged' on associated control 'dropdownlist1' for the trigger in UpdatePanel 'UpdatePanel1'...

Use selectedindexchanged in the place of OnSelectedIndexChanged for eventname of AsyncPostBackTrigger...

 <asp:UpdatePanel ID="upnlRestrictedArea" runat="server">
         <ContentTemplate>         
            <asp:Panel ID="pnlResArea" runat="server" Style="display: none" BackColor="#F9F6F4"
        Height="350px" Width="380px">
        <div class="PopUpheader">
            <span id="Span1" class="msg">Select Restricted Area</span> <span style="float: right">
                <asp:ImageButton ID="imgClose_resArea" AlternateText="Close" runat="server" ImageUrl="~/images/favicon.ico" /></span>
        </div>
        <table width="85%" class="cssform">
            <tr>
                <td>
                    Select Area :
                </td>
                <td>
                    <asp:DropDownList ID="ddlRestricted" runat="server" AutoPostBack="true" AppendDataBoundItems="true"
                        OnSelectedIndexChanged="ddlResArea_SelectedIndexChanged">
                        <asp:ListItem Text="--Select Area--" Value="0"></asp:ListItem>
                    </asp:DropDownList>
                </td>
            </tr>
        </table>
        <br />       
    </asp:Panel>
         </ContentTemplate>
         <Triggers>
           <asp:AsyncPostBackTrigger ControlID="ddlRestricted" EventName="selectedindexchanged" />
         </Triggers>
     </asp:UpdatePanel>