[racket] Artefact when cropping and scaling an image

Why does the following code generate visual artefacts along the right and
bottom
edges of the image?  How do I eliminate it?

#lang racket
(require 2htdp/image)

(define (crop-to-aspect-ratio im final-width final-height)
  (let* ([W (image-width im)]
         [H (image-height im)]
         [wh-ratio (/ final-width final-height)]
         [w (* wh-ratio H)]
         [h (/ W wh-ratio)]
         [image (if (<= W w)
                    (crop 0 (/ (- H h) 2) W h im)
                    (crop (/ (- W w) 2) 0 w H im))])
    (freeze (scale (/ final-width (image-width image))
                   image))))

(scale 5
       (crop-to-aspect-ratio
        (freeze (rectangle 259 356 'solid 'red))
        40 56))


Note that the two freeze instructions are essential -- the one in the crop
routine
 forces pixelation, which I want -- and the one in the calling code
simulates starting with a bitmap.

Thanks

Dan
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