Cant use extended classes variables or methods?

[Doush.]

I have this problem from time to time with AS3 where I extend a Class and then try call methods and variables which are public from the new class but I get errors that method or variable doesn't exist. My IDE recognises the methods and variables though.

Heres an example

//I have a base class
public class Box {

   public var id:int;

   public function somemethod() {}

}

//Then I extend the class
public class BiggerBox extends Box {}

//Then in a separate class I try this
var bigBox:BiggerBox = new BiggerBox();

trace(bigBox.id);

bigbox.somemethod(); 

When I run the script flash hates it. I thought when you extend a class it was supposed to inherit its properties and methods?

Does anyone else have these problems? I shouldn't have to cast it as Box. I swear I have this working in a project but cant see the different in setup?