Cannot make tree-view display updated values - gets error

I make a capi:tree-view like this

 
(capi:contain (make-instance 'capi:tree-view 
                             :roots (mapcar (lambda (n) (make-instance
'capi:item :data n)) '(1 2 3 4)) 
                             :callback-type :item-element 
                             :action-callback 'edit-item 
                             :print-function 'print-it)))

(defun edit-item (item element) 
  (incf (capi:item-data item)) 
  (capi:redisplay-collection-item element item))

(defun print-it (x) 
  (format nil "Value: ~A" (capi:item-data x)))

When I double-click an item, I expect its value to increase by 1, and to be
redisplayed. But I get

Error: Exception C0000005 [flags 0] at 20803BC2 
     {Offset 40 inside #<Function 1 subfunction of (TOP-LEVEL-FORM 3)
20803B9A>}
eax       56 ebx       18 ecx      306 edx  1EACC74
esp  35FE528 ebp  35FE628 esi        2 edi 20803B9A
  1 (abort) Return to event loop.
  2 Destroy interface "Container" (of type CAPI:INTERFACE)

Call to ERROR
Call to WIN32::SIGNAL-EXCEPTION-ERROR
Call to (METHOD CAPI-LIBRARY:REPRESENTATION-UPDATE-TREE-VIEW-REDISPLAY-ITEMS
(CAPI-WIN32-LIB::R-TREE-VIEW T))
Call to (METHOD CAPI::UPDATE-REPRESENTATION :AFTER (CAPI::TREE-VIEW-MIXING
T))
Call to CLOS::METHOD-COMBINATION-TEMPLATE
Call to CAPI::DO-REDISPLAY
Call to (METHOD CAPI::REDISPLAY (CAPI::PANE-WITH-CHILDREN))
Interpreted call to EDIT-ITEM
Call to CAPI::EXECUTE-CALLBACK
Call to CAPI:EXECUTE-WITH-INTERFACE
Call to (SUBFUNCTION 1 (WIN32:DEFINE-COMMON-CONTROL-FUNCTION
CAPI-WIN32-LIB::R-TREE-VIEW WIN32::NM_DBLCLK))
Call to WIN32::WND-PROC
Call to (SUBFUNCTION WIN32::|%FOREIGN-CALLABLE/wnd_proc_callback|
(FLI:DEFINE-FOREIGN-CALLABLE "wnd_proc_callback"))
Call to FLI::FOREIGN-CALLABLE-ENTRY-POINT-AUX-NESTED
Call to WIN32::CALL-WINDOW-PROC$RAW
Call to WIN32::WND-PROC

If I inspect the tree-view roots, I find that the value of the
double-clicked item has indeed been increased by 1. If I remove the call to
capi:redisplay-collection-item, then the value is increased, but not
redisplayed; the tree looks the same all the time.

This is the smallest example I've been able to find for reproducing this. I
have a capi:tree-view where some of the items are changed either by the
program, or by the user via an action and a callback like in this example. I
cannot make the tree-view display the updated value. Whatever I do I get
this exception.  I may be doing something wrong, but what? 

LWW 6.0.1


Eivind