request.getInputStream() yields unreadable stream

[Carl Hall]

I'm trying to read the raw body of a request so I can proxy it over to
another service but can't seem to read an input stream from the request.

This is the gist of my code:

protected void doPost(SlingHttpServletRequest request,
SlingHttpServletResponse response)
      throws ServletException, IOException {
    InputStream is = request.getInputStream();
    int i = is.read();  // i == -1 , always
}

Should I use a different means to get an input stream for the request body?