The second item is warning against a particular misuse of pointers that was apparently widespread at the time. On VAX Unix, it happened that the memory at address zero of a process was readable and contained zeroes. So a lot of code would misuse a NULL pointer as being equivalent to a pointer-to-an-empty-string, for instance: worked fine on the VAX!. So it's kind of a specific flavour of commandment 10. These days the common machines don't have a readable address zero, and all the old non-portable code has faded into history.