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’

Your member is declared Rigidbody, and your GetComponent is setting it to a Rigidbody2D