[plt-scheme] get-uncovered-expressions always returns () ?

I'm having no luck in persuading the coverage-tester to work in scheme/sandbox.

Specifically, I observe about this program:

#lang scheme

(require scheme/sandbox)

(define stx-placeholder #'abc)

(define to-be-evaluated
  (datum->syntax
   #f
   `(module user lang/htdp-beginner
      (require schemeunit/check)
      (define (f x) (+ 3 x)))
   stx-placeholder))

(define evaluator
  (parameterize ([sandbox-output 'string]
                 [sandbox-coverage-enabled #t])
    (make-module-evaluator
     to-be-evaluated)))

(evaluator '3)

(get-uncovered-expressions evaluator #f
                           ;#t (syntax-source stx-placeholder)
                           )

that
a) it's about the simplest use of get-uncovered-expressions, and
b) it's not working. Specifically, the call to get-uncovered-expressions
  produces (), when it appears to me that it shouldn't (in that the function 'f'
  is never called.

Please let me know if I'm mistaken about either (or both!) of these observations.

(FWIW, this is 4.2.3.9svn4jan2010; perhaps an update would help?)

John

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