[Python] "object of type 'NoneType' has no len()" error

I'm seeing weird behavior on this code:

images = dict(cover=[],second_row=[],additional_rows=[])

for pic in pictures:
if len(images['cover']) == 0:
images['cover'] = pic.path_thumb_l
elif len(images['second_row']) < 3:
images['second_row'].append(pic.path_thumb_m)
else:
images['additional_rows'].append(pic.path_thumb_s)


My web2py app gives me this error:

if len(images['cover']) == 0:
TypeError: object of type 'NoneType' has no len()​

I can't figure out what's wrong in this. Maybe some scope issue?

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