x for x in url[1:]

means each part, let path will be /index/index/index.html you will recieve empty list but only last one should be omitted.

UPD: general rule: every situation not covered in problem statement - possible.

url = list(enumerate([x for x in url[1:] \
      if 'index' not in x]))

this is bad idea

My solution falied with the same error and I'm replacing empty strings with '[no clan specified]'

UPD: look's like I have problem with column naming. Problem solved.