No problem. That's a great idea; really just to check out the docs in general.

... I think if the sample tests ...

This is a poor way to start an issue because you don't actually know.

If you had tested your code with a one-char length body, you would have seen that it would work.

Actually, with code this simple, just reeading it carefully enough is enough to figure this out.

    test.assert_equals(correct_tail("x", "x"), True)

Not a kata issue.