JML can't catch a violated precondition

Question

In my class Test I've an array of five int named a, and the method addOne(int index) that add one to the selected cell. I wrote in JML a simple precondition to control the index passed to the method. Then I try to violate this precondition calling the method with a negative index and JML can't catch this error. What's wrong?

This is Test class:

public class Test {

 public int[] a;

 public Test(){
    a = new int[]{1,1,1,1,1};
 }

 //@ requires index>=0 && index<5;
 public void addOne(int index){
    a[index]+=1;
 }
} 

And this is the Main:

public static void main(String[] args) {
    Test t = new Test();
    t.addOne(-2);
}

That throw the exception: java.lang.ArrayIndexOutOfBoundsException: -2.

With any JML message.