Ajax 自动完成扩展器引发错误

我正在使用ajax自动完成扩展器使用服务器代码加载所有国家/地区的所有城市。

一切正常..如果我点击"禁止"，将添加一组结果。第一名将是班加罗尔，第二名将是曼谷等。问题是，如果我按下键浏览曼谷和其他地方，它会抛出错误。

Javascript 运行时错误：

jaascript runtime error:System.argumentnullexception:value cannot be null.parameter name :classname in ajax autocomplete extender

.aspx

<autofill:AutoCompleteExtender BehaviorID="AutoCompleteEx" ServiceMethod="GetCompletionList"
                                        ID="fromlocation_AutoCompleteExtender" runat="server" DelimiterCharacters=""
                                        Enabled="True" ServicePath="" TargetControlID="fromlocation" UseContextKey="True"
                                        MinimumPrefixLength="2" CompletionInterval="10" EnableCaching="true" CompletionSetCount="20"
                                        CompletionListItemCssClass="autocomplete_listItem">
                                        <Animations>  
            <OnShow>  
                <Sequence>  
                    <OpacityAction Opacity="0" />  
                    <HideAction Visible="true" />  
                    <ScriptAction Script="  
                        // Cache the size and setup the initial size  
                        var behavior = $find('AutoCompleteEx');  
                        if (!behavior._height) {  
                            var target = behavior.get_completionList();  
                            behavior._height = target.offsetHeight - 2;  
                            target.style.height = '0px';  
                        }" />  
                    <Parallel Duration=".4">  
                        <FadeIn />  
                        <Length PropertyKey="height" StartValue="0" EndValueScript="$find('AutoCompleteEx')._height" />  
                    </Parallel>  
                </Sequence>  
            </OnShow>  
            <OnHide>  
                <Parallel Duration=".4">  
                    <FadeOut />  
                    <Length PropertyKey="height" StartValueScript="$find('AutoCompleteEx')._height" EndValue="0" />  
                </Parallel>  
            </OnHide>  
                                        </Animations>
                                    </autofill:AutoCompleteExtender>

C#

[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
        public static string[] GetCompletionList(string prefixText, int count, string contextKey)
        {
            SqlConnection conn = new SqlConnection(WebConfigurationManager.ConnectionStrings["connstring"].ToString());
            //SqlCommand cmd = new SqlCommand("SELECT Code,City  FROM CCode WHERE City LIKE '" + prefixText + "%'", conn);
            SqlCommand cmd = new SqlCommand("SELECT  coalesce(Code + ', ', '') + City as codes FROM CCode WHERE City LIKE '" + prefixText + "%'", conn);
            SqlDataReader oReader;
            conn.Open();
            List<string> CompletionSet = new List<string>();
            oReader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
            while (oReader.Read())
                CompletionSet.Add(oReader["codes"].ToString());
            return CompletionSet.ToArray();
        }

CompletionListItemCssClass="autocomplete_listItem"

CompletionListHighlightedItemCssClass="two"