2D Array Loop Boundary Error

What is the cause of the runtime exception in the following code segment?

public class Debug2D {
    public static void main(String[] args) {
        int[][] matrix = { {1, 2, 3}, {4, 5, 6} };
        for (int i = 0; i <= matrix.length; i++) {
            for (int j = 0; j <= matrix[i].length; j++) {
                System.out.print(matrix[i][j] + " ");
            }
            System.out.println();
        }
    }
}