UML14 — UML2 superstructur: actor

UML2 superstructure 03-08-02
p. 513
"[1] An actor can only have associations to use cases, subsystems, components and classes, and these associations must be binary."

A subsystem is a component stereotype, so it doesn't make sense to mention it here.

I would propose the following constraint instead of the above one:
"[1] An actor can only have associations to classifiers, and these associations must be binary."

It makes sense that an actor can have binary associations to the subject they are interacting with.
The subject of an use case is a classifier.

see above