Adobe Flash CC: The definition of base class Bitmap not found in custom class

Hello!

I am developing a new game and wanted to subclass the Bitmap class. I am working on Adobe Flash CC.

I have the following code:

package  com.classes{

  public class AdBitmap extends Bitmap {

       public function AdBitmap() {

            // constructor code

       }

  }

}

However, when I run, I get the following error: The definition of base class Bitmap was not found.

Any ideas?

Cheers!