[plt-scheme] How do optional parameters work

If I enter the following module into DrScheme, version 3.99.0.26-svn6jun2008
(self compiled on a ubuntu zmd64 (module language)

#lang scheme
(provide foo)
(define (foo a #:optional (b #f)) #t)

and hit check syntax

I get an internal error

path->complete-path: expects argument of type <path (for any platform) or
string>; given |expanded module|

and Dr scheme is in a confused state  with nothing showing up in the
definitions window  (it does not want to look at my code, and so it assumes
that I don't either)

I'm not sure whether I am screwed up, or this is a bug.  Probably both.
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