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Can someone help me with abstract function?

Can someone help me with abstract function?

I wrote this to find out the minimal of victory margins, but when I try to run the example on it it gave me this error:
foldr : third argument must be a list, given (lambda (a1) ...)

(define (narrow-victories members)
[(define (vic-margin amp)
(mp-victory-margin amp))
(define (map-margin members)
(map vic-margin members))]
(foldr min (mp-victory-margin (first members)) map-margin)))

I don't quite understand because when I try testing just the first two definition in local, the result came up is indeed a list of numbers.

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