Image access problem

The following code causes error "MAKE-IMAGE-ACCESS called on a non-editable image" in LWM Personal 6.0.1: 

(setf pane (capi:contain (make-instance 'capi:output-pane)))
(setf image (gp:load-image pane (gp:read-external-image my-file) :force-plain t))
(gp:make-image-access pane image)

my-file is a BMP file. I tried to replace :force-plain with :editable but without success. It worked as expected in LW 5.1 with the same file. Any ideas?

Thanks,

Michal

-- 
Michal Krupka
Dept. Computer Science
Palacky University, Olomouc
17. listopadu 12, CZ-771 46, Olomouc
Czech Republic