[racket] get-uncovered-expressions

Sorry if I am missing something obvious, by why does the following produce '() ?

(define Ev
  (parameterize ([sandbox-coverage-enabled #t])
    (make-evaluator 'lang/htdp-intermediate
                    `(define (f x)
                       (if (zero? x) "zero" "non-zero"))
                    `(f 4)
                    )))
(get-uncovered-expressions Ev)


I would have expected it to indicate somehow that the "zero" case is not covered?

--- nadeem