Negative bound in Java

I'm making a program where i calculate a random number using a function named generarAleatorio.

public static int generarAleatorio(int l){
        java.util.Random X = new java.util.Random();
        return X.nextInt(l) ;
    }

Then, I use that number to locate a value inside an array using the length of the array as the parameter for the generarAleatorio parameter

public static String generarFecha(){

int[] dias={1,2,3,4,5,6,7,8,9,10,
                    11,12,13,14,15,16,17,18,19,20,
                    21,22,23,24,25,26,27,28,29,30};

int[] mes={1,2,3,4,5,6,7,8,9,10,11,12};

return String.format("%d-%d",dias[generarAleatorio(dias.length)],
                             mes[generarAleatorio(generarAleatorio(mes.length))]);
}

The problem is that sometimes an Exception is showed that looks like this

Exception in thread "main" java.lang.IllegalArgumentException: bound must be positive
    at java.util.Random.nextInt(Random.java:388)
    at generadorregistros.GeneradorRegistros.generarAleatorio(GeneradorRegistros.java:70)
    at generadorregistros.GeneradorRegistros.generarFecha(GeneradorRegistros.java:108)
    at generadorregistros.GeneradorRegistros.main(GeneradorRegistros.java:48)
Java Result: 1
BUILD SUCCESSFUL (total time: 0 seconds)

It tells me that the Bound im sending its not positive, but, as far as i know, the length of the Array is always the same and also positive.

What's the cause of this? Is the length of the array not always positive?