[plt-scheme] post-pure-port

I'm trying to write a simple web-page-submitting script which will
post data to a URL and get the response page back. I wrote the
following function:

(define (post urlstr data)
  (html->sxml
   (post-impure-port
    (string->url urlstr)
    (string->bytes/utf-8 (alist->form-urlencoded data)))))

Invoking this with the URL of a test page I created and an assoc with
one value in it doesn't seem to actually post the data. What I get
back is an SXML representing the page as if I had invoked it without
posting any data to it. A google search found me some more complicated
code that used call/input-url which I adapted as follows:

(define (post urlstr data)
  (let ([post-data
         (parameterize ([current-alist-separator-mode 'amp])
           (string->bytes/utf-8
            (alist->form-urlencoded data)))])
    (call/input-url
     (string->url urlstr)
     (case-lambda
       [(url) (post-pure-port url post-data)]
       [(url los) (post-pure-port url post-data los)])
     html->sxml)))

This has the same behavior for me. I call it with an assoc that looks
like '((test . "test value")) and the test page at the URL understands
the 'test' argument but I get back SXML of the page without any data
posted to it.

Thanks for the help,
-mike