Your first two list comprehensions are useless, but your code seems fine apart from that. It throws an IndexError when s1 or s2 contain uppercase letters, and that's strange since the description says Only lower case letters will be used (a-z). No punctuation or digits will be included.. If you replace ord(i) with ord(i.lower())in your code, it should pass the tests.

Your method is correct. Theres a bug in tests because there are inputs with uppercase letters.

BTW you can see inputs by printing them with print(s1 + ' ' + s2).