[Python] Loading a transparent PNG in PIL gives RGB instead of RGBA

I'm trying to use PIL to composite some aerial imagery and running into some trouble. I use PIL to load this image with this little bit of code:

composite = Image.new('RGBA', (256, 256))
url = 'http://...'
resp = requests.get(url)
content = StringIO(resp.content)
image = Image.open(content)
composite.paste(image, (0, 0), image)


When I make the call to composite.paste(), PIL gives me the error "ValueError: bad transparency mask". When I print image.mode, sure enough it's simply RGB instead of the expected RGBA (which paste() requires).

Where does the alpha channel on my downloaded PNG go?

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