No ASCII character for the number 0?

Objective-C Programming (2nd Edition) 39. C Strings
1

In the char section of this lesson it says, Nor is there an ASCII character for the number 0.

But the code exercise in that section prints out 0 in the set of ASCII characters:

30 is 0

So I’m a bit confused by the statement that, “There is no ASCII character for the number 0”.