Rookie Scheme Mistake

I’m currently reading Structure and Interpretation of Computer Programs and during the first minutes of trying my luck into scheme I faced the following error:

; application: not a procedure;
;  expected a procedure that can be applied to arguments
;   given: 5

After googling for a while without success I’ve realized that my rookie mistake. Instead of using the super friendly Polish Notation, I was using the usual Infix notation.

I had this:

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5
(define (square n)
  (n * n)
)

(square 5)

When I should be doing this:

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(define (square n)
  (* n n)
)

(square 5)

Problem solved :D