Object reference not set to an instance of an object PlayerController.FixedUpdate ()

Hi
This is my first game in Unity Roll A Ball. I have added the following code snippet.
public class PlayerController : MonoBehaviour {

private Rigidbody rb;

void start()
{
    rb = GetComponent<Rigidbody>();
}

void FixedUpdate()
{
    float moveHorizontal = Input.GetAxis("Horizontal");
    float moveVertical = Input.GetAxis("Vertical");

    Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
    //Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

    rb.AddForce(movement);
}

}

Have you added rigidbody to the ball???