How to get GWorld's pixel address

[minagawa@st.rim.or.jp (Minagawa,Kazushi)]

I'm having a problem with getting GWorld's pixel address using the
GetPixBaseAddr trap.  I must be doing something wrong.  

Here's the code. 

(defun access-to-gworld ()
        (rlet ((gw-p :ointer)
                  (gw-r :rect
                            :topleft #@(0 0)
                            :bottomright #@(100 100)))
    (#_NewGWorld gw-p 32 gw-r (%null-ptr) (%null-ptr) 0)
    (with-macptrs ((pix-addr (#_GetPixBaseAddr (#_GetGWorldPxMap gw-p))))
      (print (%get-byte pix-addr)))))

With macsbug I show that the error is "bus error". This means the address I
got using the GetPixBaseAddress trap is invalid.

Does anybody have any ideas about what is wrong here?

Thanks,

Minagawa,Kazushi
Nikon Systems, Inc.
Yokohama-city, Japan.