Why is my code rejected? I believe it's 💯% correct! (SOLVED)

Question

It's from a challenge "extra terrestrials"
https://code.sololearn.com/cmG14rRZ7v1c/?ref=app

Answer 1

What happens if len == 0?

Answer 2

Not all characters are printable, so even though you can't see the difference, the computer can.

Answer 3

Hah 😅 Dennis, you nailed it, I didn't even notice!