0 is a debated natural number and can be a member of the natural number set depending on which set you are using. See here http://oeis.org/A000027.

N = {0, 1, 2, 3, ...}         - nonnegative integers

N = (1, 2, 3, 4, ...}         - positive integers

However, I agree with you on the fact it should state that it is including 0 as a natural number.