why function allowBlank:false, on my fieldset is not working ? this is my code :

form with fieldset :
Code:
...
    formReplyWeb = new Ext.FormPanel({
        frame: true,
        baseCls: 'x-panel',
        labelAlign: 'left',
        labelWidth: 100,
        url: '/TA/data/webdata.jsp?type=3&id='+selectedWebDetailId,
        method: 'POST',
        waitMsgTarget: true,
        items: [
            new Ext.form.FieldSet({
                title: 'Tulis pesan baru',
                iconCls: 'webEdit',
                autoHeight: true,
                autoWidth: true,
                buttonAlign:'right',
                defaultType: 'textfield',
                items: [
                    webSender,
                    webSubject,
                    webProductID,
                    webCreatedDate,
                    webMessage,
                    webReply        // <----- this is the html editor field
                ],
                buttons: [{
                    text: 'Kirim',
                    handler: sendWEB
                },{
                    text: 'Reset',
                    handler : resetWebFSData
                },{
                    text: 'Tutup',
                    handler : function() {winReplyWeb.hide();}
                }]
            })
        ]
    });
...
this is the html editor as an input field :

Code:
...
var webReply = new Ext.form.HtmlEditor({
        id:'webReply',
        name: 'webReply',
        fieldLabel: 'Pesan balasan ',
        width:500,
        height:200,
        allowBlank: false,                // <--------------- not working / functioning
        enableFormat:false,
        autocomplete:'off',
        enableSourceEdit: false,
        enableColors : false,
        enableFont : false,
        //labelWidth: 5,
        labelSeparator:''
    });
...
what wrong with that ??