[Python] Root mean square difference between two images using Python and PIL

I need to have a function like the one found here: http://effbot.org/zone/pil-comparing-images.htm that calculates the root mean square difference between two images. The code looks like this:

import ImageChops
import math, operator
from functools import reduce

def rmsdiff(im1, im2):
"Calculate the root-mean-square difference between two images"

h = ImageChops.difference(im1, im2).histogram()

# calculate rms
return math.sqrt(reduce(operator.add,
map(lambda h, i: h*(i**2), h, range(256))
) / (float(im1.size[0]) * im1.size[1]))


Trying to run this code leads to the following error:

TypeError: unsupported operand type(s) for ** or pow(): 'NoneType' and 'int'.

What's the matter with it?

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