[cairo] Can't draw more

[Gary Jaffe]

I'm new to cairo, and I wrote a small pygtk application that uses cairo 
to draw a background on a gtk.DrawingArea.  Drawing the background works 
fine, but I also have a button that when clicked does some more drawing 
on the background.  When I execute the context.stroke() statement 
following the button click, I get the following error.

cairo.Error: the target surface has been finished

Obviously, I'm missing something here.  Can someone please clue me in on 
how to draw more on the background after the background is already drawn 
or point me to some documentation that will tell me?

Thanks,
Gary