Referencing an interface inner class from implementor doesn't work

class Usage implements Koo {	
  static class MyInner extends Inner {}
}

public interface Koo {

    class Inner {
    }

}

startup failed:
Script1.groovy: 2: unable to resolve class Inner 
 @ line 2, column 30.
   class Usage implements Koo {	static class MyInner extends Inner {}
                                ^

1 error