Asp.Net自动完成set_contextKey＆ldquo;对象不支持此属性或方法＆rdquo;

<asp:TextBox ID="txtProduct" runat="server" AutoPostBack="true" OnTextChanged="txtProduct_TextChanged" Width="181px" /><ajaxToolkit:AutoCompleteExtender
    ID="AutoCompleteExtender_txtProduct" BehaviorID="acExt" runat="server" TargetControlID="txtProduct"
    CompletionListCssClass="autocomplete_completionListElement" CompletionListItemCssClass="autocomplete_listItem"
    CompletionListHighlightedItemCssClass="autocomplete_highlightedListItem" MinimumPrefixLength="2"
    CompletionInterval="1000" ServicePath="~/WS/Service.svc" ServiceMethod="GetProductsByName"
    EnableCaching="true" DelimiterCharacters=";" UseContextKey="true" OnClientItemSelected="txtProduct_ClientItemSelected">
</ajaxToolkit:AutoCompleteExtender>

function ddlStore_onchange() {
    $('#acExt').set_contextKey($('#<%= ddlStore.ClientID %>').val());
}