ActionScript 3.0 :: Can't Access Properties Of Extended Abstract Classes

Im building a project using an MVC pattern. I have (pseudo) abstract classes Model and View, each extended with particular models and views. But I cannot access properties on the extended classes of Model...Model simply distributes XML data, heres its constructor:

Code:
public function Model(data:XML)
{
_data = data; /*_data is a protected var of type XML in Model*/
}

MainModel extends Model, and has a property called _background of type Background which also extends Model:

Code:

private var _background:Background = new Background(some XML source);
public function MainModel(data:XML)
{

[code]....

The problem arises when I try to pass MainModel's background property as a parameter of _bgView (highlighted in red above). The compiler says there is no background property on Model, which is true, but I need it to look at its extended class MainModel, not Model. I've typed MainView's constructor parameter with MainModel, but it is still seeing it as Model.