Ошибка NullReferenceException: Object reference not set to an instance of an object как исправить

проблема заключается в rb.velocity я не знаю как исправить ошибку.

public class PlayerController : MonoBehaviour

public float speed;
public float jumpForce;
private float moveInput;

private Rigidbody2D rb;

private bool facingRight = true;

private bool isGrounded;
public Transform feetPos;
public float checkRadius;
public LayerMask whatIsGround;

private void start()
{
    rb = GetComponent<Rigidbody2D>();
}

private void FixedUpdate()
{
    moveInput = Input.GetAxis("Horizontal");
    rb.velocity = new Vector2(moveInput * speed, rb.velocity.y); //rb.velocity равен null

    if (facingRight == false && moveInput > 0) 
    {
        Flip();
    }
    else if (facingRight == true && moveInput < 0)
    {
        Flip();
    }
}

private void Update()
{
    isGrounded = Physics2D.OverlapCircle(feetPos.position, checkRadius, whatIsGround);

    if (isGrounded == true && moveInput.Equals(KeyCode.Space))
    
    rb.velocity = Vector2.up * jumpForce;
}

void Flip()
{
    facingRight = !facingRight;
    Vector3 Scaler = transform.localScale;
    Scaler.x *= -1;
    transform.localScale = Scaler; 
}

}