[plt-scheme] Bad interaction w/Unix

This is on Mac OS X.  I have a shell script:
#! /bin/sh
#|
exec mzscheme -qu "$0" ${1+"$@"}
|#
(module client mzscheme
   (require (lib "md5.ss"))
   (define test-string (make-string 100000))
   (define (md-port-chunks port chunk-size)
       (let loop ()
	   (begin
	       (display (md5 (read-bytes chunk-size port)))
	       (newline))
	   (if (not (eof-object? (peek-byte port)))
	       (loop)))))
(md-port-chunks (open-input-string test-string) 1024)

When I invoke it, I get this:
./md5.sh
default-load-handler: expected a `module' declaration for `md5',
found: declaration for `client' in: #<path:/Users/jmc/src/Scheme/
md5.sh>

What is this telling me, and what should I do about it?
Perhaps it's a mismatched-parentheses problem, it's hard to tell
because of the mixing of shell-mode and scheme-mode.