I'm looking to set a leader from an array of sprites, I keep on getting a NullReferenceException was unhandled error from within my main game class when calling the UpdateMouse Method. What have I dont wrong here?

class MouseSprite
{


    Random random = new Random();
    private MouseSprite leader;

    public void UpdateBoundaryBox()
    {
        mouseBounds.X = (int)mousePosition.X - mouseTexture.Width / 2; mouseBounds.Y = (int)mousePosition.Y - mouseTexture.Height / 2;
    }

    public void UpdateMouse(Vector2 position, MouseSprite [] mice, int numberMice, int index)
    {


        Vector2 catPosition = position;
        int enemies = numberMice;
        this.alive = true;

        mice[random.Next(0, mice.Length)] = leader;