[4.1.0] Doesn't Work Mask on TextField

[Just-Boris]

I try to mask combobox:

    // The data store containing the list of states
    var states = Ext.create('Ext.data.Store', {
        fields: ['abbr', 'name'],
        data : [
            {"abbr":"AL", "name":"Alabama"},
            {"abbr":"AK", "name":"Alaska"},
            {"abbr":"AZ", "name":"Arizona"}
            //...
        ]
    });
    Ext.create('Ext.panel.Panel', {
        items:[
            this.textfield = new Ext.form.ComboBox({
                fieldLabel: 'Choose State',
                store: states,
                queryMode: 'local',
                displayField: 'name',
                valueField: 'abbr'
            }), {
                xtype: 'button',
                text:'clickme',
                handler: function() {
                    this.textfield.getEl().mask();
                    alert('done');
                },
                scope: this
            }
        ],
        renderTo:'container'
    });

That's works in Chrome, Firefox and Opera. But in IE8 or IE9 that's doesn't work. Appears error message: "SCRIPT5007: Unable to get value of the property 'dom': object is null or undefined" - ext-all-debug.js, line 14035 character 9
In ExtJS v 4.0.7 this code works correctrly, without errors.

[Just-Boris]

I workaround this bug using disable() without mask().