Basic understanding of how classes are working

1

i am working with flashDevelop, following a actionscript book and trying to make a simple test (that is not working):
this is my document class:

package  
{
import flash.display.Sprite;
    public class SubClass extends SuperClass
    {        
        public function SubClass() 
        {
            trace(SuperClass.className);            
        }        
    }
}

and this is the SuperClass:

package  
{
    public class SuperClass 
    {
        static public var className:String = "SuperClass";        
    }
}

and i get an error :
exception, information=TypeError: Error #2023: Class SubClass$ must inherit from Sprite to link to the root.