Python PIL - Draw Circle

I am trying to draw a simple circle and save this to a file using the Python Imaging Library:

import Image, ImageDraw

image = Image.new('RGBA', (200, 200))
draw = ImageDraw.Draw(image)
draw.ellipse((20, 180, 180, 20), fill = 'blue', outline ='blue')
draw.point((100, 100), 'red')
image.save('test.png')

The point draw.point appears on the image, but the ellipse itself does not. I tried changing the mode to just RGB (I thought the mode might affect what is displayed), but this did not solve it.

How can I fix this? Thanks!