[PyMOL] Trouble making pictures in a script

Hi,

I have a Python script that sets a system up and then calls cmd.png()
to render an image. I use it to process several files at a time, and
there's no need to look at it while it's working, so I use

pymol -qcr myscript.py

to launch it. It works fine when I ray trace things, but if I just set
the system up and call cmd.png(), I get this error:

 ScenePNG-WARNING: invalid context or no image.

Calling cmd.draw() instead of cmd.ray() doesn't seem to fix things.
Any ideas? Some of the systems are big enough that cmd.ray() fails, so
I'd like to be able to find a workaround.

Thanks,

-michael

-- 
Biophysics Graduate Student
Carlson Lab, University of Michigan
http://www.umich.edu/~mlerner