The class "clade" is void

The problem with the class clade is that it is void. This fact did Willi Hennig try to turn up-side-down by asserting that "only clades are natural groups". His assertion can of course not be denied, since the class "natural group" lacks definition, but it can just as of course not make the class not void. Instead, the class clade is void independently of whether Hennig thought (and cladists think) it's "natural" or not.