valueNotFoundText disappears

Hi!

I have in my application a combo configured like this:

Code:

          fieldLabel: 'Class',
          name: 'class_id',
          xtype: 'combo',
          columnWidth: 0.5,
          store: Ext.data.StoreManager.lookup('Classes'),
          valueNotFoundText: '--Select a class--',
          queryMode: 'local',
          valueField: 'id',
          displayField: 'name',
          forceSelection: true,
          grow: true,
          editable: false,

Everything works as expected, save one thing: valueNotFoundText disappears if I collapse the combo without selecting any value. I would like it to remain there until the user selects one of the available options.

I am using 4.1.3. Any ideas?

Thanks in advance!