To null check or not to do a null check?

To null check or not to do a null check?

Here is a code authored by Josh bloch, ( Linkedlist.java)
* @throws NullPointerException if the specified collection is null
*/
public boolean addAll(int index, Collection<? extends E> c) {
checkPositionIndex(index);
Object[] a = c.toArray();
int numNew = a.length;
if (numNew == 0)
return false;
Node<E> pred, succ;
if (index == size) {
succ = null;
pred = last;
} else {
succ = node(index);
pred = succ.prev;
}
Here I dont see any null ptr check for Collection c. On contrary effective
java very much stresses on parameter validation, emphasizing null pointer
check. If an invalid parameter value is passed to a method and the method
checks its parameters before execution, it will fail quickly and cleanly
with an appropriate exception.
I need to know what I am missing ? In other words why did he not do a null
check for addAll function ?