Gimp 2.4.1 and Script-Fu: set variable gives "unbound variable"

My script starts with the following code:

(define (script-fu-scale-in-steps-to-screen image drawable)

;get original size of image
	(set! actwidth (car (gimp-image-width image)))
	(set! actheight (car (gimp-image-height image)))

	(set! ratio 1.05)

When I try to run it in Gimp 2.4.1, it gives the error 

Error while executing (script-fu-scale-in-steps-to-screen 1 2)
Error: set!: unbound variable actwidth

If I remove the first set!, I get the same error message for actheight.
My first thought was that maybe gimp-image-width/height have been
renamed, but when I remove first and second set, I also get the
message for ratio).
So it looks to me as if the set! is the problem.

The script ran without error messages in Gimp 2.2.x.

Can anyone help me repair my script so I can use it in 2.4.1?

Thanks in advance!

Andreas

PS: 
Don't know whether it is important, but the script is registered with
this code:

(script-fu-register
 "script-fu-scale-in-steps-to-screen"
 _"<Image>/Image/Scale in Steps to Screen ..."
 "Scale the image from its original size to the new size in several steps"
 "Scale it"
 "Andreas Waechter <[email protected]>"
 "2007"
 ""
 SF-IMAGE "Image" 0
 SF-DRAWABLE "Drawable" 0
)







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