DefaultStyleLabel error

I'm using thi code

var oFCKeditor2 = new FCKeditor('mTesto',0,500 ) ;
    oFCKeditor2.BasePath = '/fckeditor/';
    oFCKeditor2.Config['EnterMode'] = "div"
    oFCKeditor2.Config['DefaultStyleLabel'] = 'Red Title';
    oFCKeditor2.ReplaceTextarea() ;

to set the default style to use for write in editor

the only effect is to set the value of combo box but then text is not affected from the style until I don't select manually the style from the combo

what is my error