[plt-dev] scribble with gui

Hi all,

I am writing a scribble document and part of the source code is:

@(define my-eval
   (call-with-trusted-sandbox-configuration
    (lambda ()
      (parameterize ((sandbox-output 'string)
                     (sandbox-error-output 'string))
        (make-evaluator 'scheme/base #:requires
                        (list (if gui?
                                  'scheme/gui/base
                                  'scheme/base)))))))

@examples[#:eval my-eval
          #|code that relies on scheme/gui/base|#]

@(kill-evaluator my-eval)

I put the scrbl file into a .plt file, and then install it inside 
DrScheme (which I assume will make gui available).
But the generated doc suggests otherwise:

The "@(define" part leaves nothing in the html file, which is right.

The "@examples" part generates, after the input:

  reference to an identifier before its definition:

  image in module: 'program


in red, which I assume is caused because 'scheme/gui/base is not 
required. Is this a bug, or did I do something wrong?

More weirdly, after that the html file contains a line:

#<void>

It seems to be the result of "@(kill-evaluator", but shouldn't that not 
be printed?

Thanks,
Chongkai