The Five Skandhas are the general characteristics of the Dharma, however, the Six perceptual faculties (i.e. Six Roots, or Six Internal Bases), the six objects of perceptions (i.e. Six Dusts, or Six External Bases) and also the Six Consciousness are the special characteristics of the Dharma. As the general characteristics are empty, their special characteristics must be non-existent too.
The Six Internal Bases and the Six External Bases are together called the Twelve Bases. Base implies the meaning of germinating and nourishing. All mental activities are germinated and nourished from these Twelve Bases.