Thank you for informing me of this... I was not aware of that before hand. Anyways, someone else typed that on my account but thanks nevertheless.

No, 0 can be a natural number depending on the definition (which, in set theory usage, almost always includes 0).

Besides, the description already says if either negative or decimal return check input, which 0 is not one of them.

0 is not a natural number. A natural number is defined as a positive integer. In the event 0 is passed in the correct output based on the prompt should be "check input"

It worked fine for me