[plt-scheme] custodian + process/ports

Hello,

How can a process created with process/ports be properly terminated (without creating zombie 
processes) when its stdin is closed by a custodian?

I.e.: closing gnuplot's stdin produced by the following code creates a defunct process.

(define (open-gnuplot)
  (match (process/ports (current-output-port) #f
                        (current-output-port) "gnuplot")
         ((list stdout
                stdin
                pid
                stderr
                control)
          stdin)))


Cheers,
Tom