How to add context menu actions?

[Ambrevar]

This might be more of a GTK question than a Common Lisp question, but allow me to post here, just in case.
I'm trying to create a custom context menu in WebKitGTK (using cl-webkit).

A simple attempt:

; Helper to populate the browser "context-menu-entries" slot:
(defun make-context-menu-entries ()
	(let ((foo-action (gio:g-simple-action-new
										 "foo"
										 (glib:g-variant-type-new glib:+g-variant-type-maybe+))))

		(gobject:g-signal-connect
		 foo-action "activate"
		 (lambda (action parameter)
			 (declare (ignore action parameter))
			 (log:info "foo action")))

		(gio:g-simple-action-set-enabled foo-action t)

		(list :foo
					(webkit:webkit-context-menu-item-new-from-gaction
					 foo-action
					 "Do foo"
					 (cffi:null-pointer)))))

; Initialize the web view:
; ...
(gobject:g-signal-connect (gtk-object buffer) "context-menu"

	 (lambda (web-view context-menu event hit-test-result)
		 (declare (ignore web-view event hit-test-result))

		 (webkit:webkit-context-menu-append
			context-menu
			(getf (context-menu-entries *browser*) :foo))
		 ;; Return nil to show the context menu.
		 nil))

It works and shows "Do foo" in the context menu; sadly, it's greyed out.
It seems that g-simple-action-set-enabled does not do what I
think it does.

Any clue what I'm missing?