UMLR — Generalization should be limited to relate similar UML-elements

The Generalization relationship can be used between instances of all subclasses of the metaclass Classifier. At least in the chapter on Generalization there is no constraint given. Thus it is allowed to have a generalization between an Activity and a signal. This definitely makes no sense. Does it make sense to have a generalization between an Activity and a Class? What does that mean for the instances?

The only Metaclass that defines a constraint for Generalizations seems to be Stereotype (page 293).

Tools seem to enforce, that only instances of Metaclasses, that are in a Generalization relationship on the meta level may have Generalization relationships on the model level. I'm not sure, whether this makes sense. Anyway, I couldn't find anything in the specification supporting this view.

Suggestion:
Add a constraint to Generalization, that limits the related elements to be of the same or compatible Metaclasses.