ActionScript 3.0 :: Set Up A System Polymorphism Not Working

Essentially I want to set up a system where a I have a bunch of subclasses that all end up doing the same thing, so I declare a base class that has some pretty basic functions. Here is my code:

Code:
// BaseClass.as
package
{
import flash.display.MovieClip;

[Code]...

Each class that will extend the BaseClass has a 'mouth' movieclip inside of it, though I have not set up a movieclip for the BaseClass. It looks good in theory, but of course I get an error:

Code:BaseClass, 1120: Access of undefined property mouth 'Automatically declare stage instances' is checked, so I'm not sure what else to do here.