Ext window 的遮罩没有起作用

[code="java"]
var win = new Ext.Window({

title: '手机短信报警',

width: 300,

height:130,
id:'win',
plain : true,

closeAction: 'hide',
maskDisabled : false ,

items:new Ext.form.FormPanel({
width:290,
maskDisabled : false ,
frame:true,

items:[{
xtype:"panel",
layout:"column",
fieldLabel:"手机号码",//文本框标题
isFormField:true,
items:[{

columnWidth:.5,
xtype:"textfield",
allowBlank:false,
blankText:"不能为空，请填写",
name:"number",
id:"number",
anchor:"60%"
},{

             },{}]
        },{
             xtype:"panel",
             layout:"column",
             fieldLabel:"验证码",     
             items:[{
                 columnWidth:.3,
                 xtype:"textfield",
                 allowBlank:false,
                 blankText:"不能为空，请填写",
                 name:"code",
                 id:"code",
                 anchor:"60%"
             },{
                 xtype:"button",
                 anchor:"40%",
                 text:"发送短信",
                 id:"send",
                 handler:function(){
                     checkNmber();
                } 
             },{
                 xtype:"label",
                 id:"time"
             }]

        }],   
         buttons:[{text:"提交",id:'submit',handler:function(){validate()}},{text:"重置",handler:function(){alert("事件！");}}]
    })   
});
win.show();

[/code]

不管maskDisabled 等于true还是false都没有遮罩效果

modal:true

ok .