[plt-scheme] sandbox & ports closed prematurely

Hello,

I ran into strange sandbox evaluator behaviour where an output port gets closed immediately, 
instead of at evaluator destruction time:

Welcome to MzScheme v4.0.1.3 [3m], Copyright (c) 2004-2008 PLT Scheme Inc.
> (require scheme/sandbox)
> (sandbox-security-guard (current-security-guard))
> (define e (make-evaluator 'scheme/base))
> (define f (e '(open-output-file "/tmp/bla")))
> (display 123 f)
display: output port is closed

 === context ===
/usr/local/plt/collects/scheme/private/misc.ss:68:7

Tested with last night's build.

Cheers,
Tom