[plt-scheme] profiling doesn't show any data -- why?

This is on Debian Sarge 
(Linux debian 2.4.27-2-686 #1 Mon May 16 17:03:22 JST 2005 i686 GNU/Linux)

$ mzscheme --version
Welcome to MzScheme version 301.4, Copyright (c) 2004-2006 PLT Scheme Inc.

$ time mzscheme  -qu profile.scm
832040
Sorting profile data...
Total samples: 0

real	0m0.481s
user	0m0.420s
sys	0m0.020s
Total samples: 0

I expected to see a large number after "samples", and a list with one
entry for "fib" which says something like "called 832040 times".

As you can see, the program runs for a reasonably long time, long
enough, I expect, for some samples to be collected.

I assume I'm missing something obvious, but (obviously!) haven't
figured out what it is.

    (module profile
    mzscheme
    (require (lib "errortrace.ss" "errortrace"))

    (profiling-enabled #t)
    (profiling-record-enabled #t)
    (profile-paths-enabled #t)

    (define (fib n)
      (cond
       ((zero? n)
        0 )
       ((= n 1)
        1)
       (else
        (+ (fib (- n 1))
           (fib (- n 2))))))

    (display (fib 30))
    (newline)
    (output-profile-results #t #t)
    )

-- 
Paul Graham is right.
        --Shriram Krishnamurthi