issue with where filter

Graph g = TinkerGraph.open();

g.addVertex(T.label,"GROUP","name","Acme");

List<Vertex> list = g.traversal()
.V()
.hasLabel("GROUP")
.where(__.outE().hasLabel("PART_OF").count().is(0))
.toList();

Assert.assertEquals(1, list.size()); // actual size is 0