capi:text-input-range in toolbar

I have problems using a capi:text-input-range in a window toolbar (LW  
6.0.1 on Mac OS X 10.5).

In the interface definition I use something like:
(my-choice :accessor my-choice
            :initform (make-instance 'capi:text-input-range
                                     :title "My Title"
                                     :start 0
                                     :end 200
                                     :value 100
                                     :callback 'my-callback
                                     :callback-type :interface
                                     :enabled nil)
            :type capi:text-input-range)

Then I have a creation callback for the interface, something like this:
(defun my-interface-create-callback (interface)
   ;; toolbar buttons
   (setf (capi:interface-toolbar-items interface)
         (list (my-choice interface))))

When the creation callback is run, I get an error saying:
No applicable methods for #<STANDARD-GENERIC-FUNCTION CAPI-COCOA- 
LIBRARY::REPRESENTATION-OUTERMOST-VIEW 21095A42 with args (NIL)>

If I instead use a capi:text-input-pane in the interface definition,  
the interface is nicely displayed:
(my-choice :accessor my-choice
               :initform (make-instance 'capi:text-input-pane
                                        :title "My Title"
                                        :text "100"
                                        :callback 'my-callback
                                        :callback-type :interface)
               :type capi:text-input-pane)

Thanks!
Sven