Strange UpdatePanel and Extender control error - asp.net

Question

I have the below markup in a master page, and when I access a page using that master, I get the error:
An extender can't be in a different UpdatePanel than the control it
extends.
<asp:UpdatePanel ID="UpdatePanel99" runat="server">
<ContentTemplate>
<script type="text/javascript">
function OnAutoCompleteSelected(source, eventArgs) {
$("#autoCompleteHidden").val(eventArgs._text);
__doPostBack("autoCompleteHidden", "");
}
</script>
<table>
<tr>
<td style="height: 25px">Search By
</td>
<td>
<asp:DropDownList ID="ddlSearchBy" runat="server" OnSelectedIndexChanged="ddlSearchBy_SelectedIndexChanged"
AutoPostBack="True">
<asp:ListItem>Emp No</asp:ListItem>
<asp:ListItem>ID No</asp:ListItem>
<asp:ListItem>Surname</asp:ListItem>
</asp:DropDownList>
</td>
<td>
<asp:ImageButton ID="imgBtnSearch" runat="server" Height="20px" ImageUrl="~/Images/Apps/mySearch2.png"
Width="20px" OnClick="imgBtnSearch_Click" />
</td>
</tr>
<tr>
<td colspan="3" style="height: 25px; margin-left: 80px;">
<div>
<asp:HiddenField runat="server" ID="autoCompleteHidden" OnValueChanged="autoCompleteHidden_ValueChanged" />
<asp:TextBox ID="txtSearchField" runat="server" Width="200px"></asp:TextBox>
<ajaxToolkit:AutoCompleteExtender ID="AutoCompleteExtender2" runat="server" TargetControlID="txtSearchField"
ServicePath="~/AutoComplete.asmx" ServiceMethod="GetEmployees" Enabled="False" CompletionInterval="100"
MinimumPrefixLength="1" CompletionSetCount="10000" OnClientItemSelected="OnAutoCompleteSelected"
CompletionListCssClass="CompletionList" CompletionListItemCssClass="CompletionListItem" CompletionListHighlightedItemCssClass="CompletionListHighlightedItem">
</ajaxToolkit:AutoCompleteExtender>
</div>
</td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
There is only one UpdatePanel on the master page, and the extended control, txtSearchField is contained well within it only. What am I doing wrong?

Answer 1

I found another AutoCompleteExtender for the same textbox outside of the UpdatePanel.