Including zero in the set of natural numbers is a matter over which mathematicians disagree. While Dictionary.com includes zero in the set of natural numbers, the University of Iowa specifies natural numbers as a set beginning with one. The university includes zero in the set of whole numbers.
The argument over whether to include zero rests on whether natural numbers are all positive whole numbers or all non-negative whole numbers. Generally, natural numbers are the counting numbers, which also calls to question if the starting point is zero or one. Dictionary.com says it includes the numbers used to count the members of a set. It includes zero, because there are zero members in an empty set.