javascript object error错误

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="ComboBoxTest.aspx.cs" Inherits="ExtjsTest.ComboBoxTest" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<script type="text/javascript" src="../../Scripts/Ext/adapter/ext/ext-base.js"></script>

<script type="text/javascript" src="../../Scripts/Ext/ext-all-debug.js"></script>



<link rel="stylesheet" type="text/css" href="../../Scripts/Ext/resources/css/ext-all.css" />
<script type="text/javascript">
Ext.onReady(function () {
var cities = [
[1, "长沙市"],
[2, "株洲市"],
[3, "湘潭市"],
[4, "邵阳市"]
];
var proxy = new Ext.data.MemoryProxy(cities);
var City = Ext.data.Record.create([
{ name: "cid", type: "int", mapping: 0 },
{ name: "cname", type: "string", mapping: 1 }
]);
//var reader = new Ext.data.ArrayReader({}, City);
var reader = new Ext.data.ArrayReader({}, [
{ name: "cid", type: "int", mapping: 0 },
{ name: "cname", type: "string", mapping: 1 }
]);
var store = new Ext.data.Store({
proxy: proxy,
reader: reader,
autoLoad: true //即时加载数据
});
//store.load();
var combobox = new Ext.form.ComboBox({
renderTo: Ext.getBody(),
triggerAction: "all",
store: store,
displayField: "cname",
valueField: "cid",
mode: "local",
emptyText: "请选择湖南城市"
});

var btn = new Ext.Button({
text: "列表框的值",
renderTo: Ext.getBody(),
handler: function () {
Ext.Msg.alert("值", "实际值：" + combobox.getValue() + "；显示值：" +
combobox.getRawValue());
}
});
})
</script>
</head>
<body >
<form id="form1" runat="server">
<div>

</div>
</form>
</body>
</html>
调试extjs程序 弹出 [object error]错误 不知道为什么