Binding the mouse scroll wheel to page scrolling

[Brian Eable]

I was trying to make the mouse scroll wheel scroll the article buffer
up and down. Here is what I put in .gnus:

(define-key gnus-summary-mode-map [mouse-4] 'gnus-summary-prev-page)
(define-key gnus-summary-mode-map [mouse-5] 'gnus-summary-next-page)

C-h k reports that the events are bound to the functions in the
summary buffer, but the article doesn't scroll. Instead, the visual
bell flashes. What did I do wrong?

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