getVertexGroup(index): unknown return type `const DzVertexGroup*'

I have two selection sets of vertices in the Geometry Editor. How can I obtain list of vertices in one of selection set? I tested this:

var oMesh = Scene.getPrimarySelection().getObject().getCurrentShape().getGeometry();
print(oMesh.getNumVertices()); // returns 4479 in my case
print(oMesh.getNumVertexGroups()); // returns 2, which is correct
print(oMesh.getVertexGroup(1).count()); // script crashes

but I get error message: TypeError: cannot call getVertexGroup(): unknown return type `const DzVertexGroup*' (register the type with qScriptRegisterMetaType())

Selected node is DzSkeleton.

The getVertexGroup is documented here.