My GetComponent is not working.

public class Bird : MonoBehaviour {

 public float upForce = 200f;

 private bool isDead = false;
 private Rigidbody rb2d;

 // Use this for initialization
 void Start ()
 {
     rb2d = GetComponent<Rigidbody2D>();
 }
 
 // Update is called once per frame
 void Update ()
 {
     if (isDead == false)
     {
         if (Input.GetMouseButtonDown (0))
         {
             rb2d.velocity = Vector2.zero;
             rb2d.AddForce(new Vector2(0, upForce));
         }
     }
 }

}

I need help, it keeps giving me the same message, CS0029 C# Cannot implicitly convert type to 'UnityEngine.Rigidbody'