[plt-scheme] Error using sandbox

Hi,

I'm trying to use the sandbox to document some code.  I have the
following in my Scribble document:

@(require scheme/sandbox)
@(define schemeunit-eval
    (make-evaluator 'scheme/base
                    #:requires '(planet "test.ss" ("schematics"
"schemeunit.plt" 3 0))))


I then get this error:

 [Running /Users/noel/programming/planet/schematics/schemeunit.plt/3/0/doc/schemeunit.scrbl]
get-module-code: cannot use extension file;
#<path:/usr/local/plt/collects/planet/compiled/native/i386-macosx/3m/main_ss.dylib>

 === context ===
core
/usr/local/plt/collects/scheme/sandbox.ss:173:2: loop
/usr/local/plt/collects/scheme/sandbox.ss:137:0: module-specs->path-permissions
/usr/local/plt/collects/scheme/sandbox.ss:422:0: make-evaluator*
/usr/local/plt/collects/setup/scribble.ss:368:9
/usr/local/plt/collects/setup/scribble.ss:47:0: setup-scribblings


This file does not exist, nor do any of the directories up to (but not
including) compiled.

I have no idea what is wrong -- it's the first time I've used the sandbox.

PLT Scheme is most recent svn (3.99.0.20), just installed (make clean
; ./configure --prefix=/usr/local/plt ; make ; make install)

Thanks,
Noel