[plt-scheme] viewport graphics problem

When I use the viewport graphics features like so:

     (require (lib "graphics.ss" "graphics"))
     (open-graphics)

     (define w (open-viewport "test" 16 16))

     ((draw-pixel w) (make-posn 7 7))
     ((clear-pixel w) (make-posn 7 7))

I get this error:

     Welcome to DrScheme, version 207.
     Language: Pretty Big (includes MrEd and Advanced).

     . procedure clear-pixel-viewport: expects 2 arguments, given 1: 
#<struct:posn>


In short, I can't seem to get clear-pixel to work as documented.  What 
am I doing wrong?

Regards,
jeh