Yea, but we now know, because the input is string and not string? that the input isn't null ;)

by the instruction...
the string input is not null!

"Implement a function that determines whether a string that contains only letters is an isogram."

...
a string that contains only letters
...

What would you expect for null? If you get null instead of a string, it's the most logical and correct behavior.

I know but the tests doesn't want me to check for null. So the tests are wrong or at least ignore this case :)