Force is not applied to the rigidbody?

using UnityEngine;
using System.Collections;

public class PlayerGunController : MonoBehaviour 
{
	public GameObject bullet;
	public int speed;
	public Transform shotSpawn;
	public float fireRate;
	
	private float nextFire;

	void Update () 
	{
		if(Input.GetMouseButtonDown(0) && Time.time > nextFire)
		{
			nextFire = Time.time + fireRate;
			Instantiate(bullet, shotSpawn.position, shotSpawn.rotation);
			bullet.rigidbody.AddForce(Vector3.forward * speed);
		}
	}
}

This is the script I am using to fire a gun. “bullet” is a sphere, with a rigidbody component. The Rigidbody has useGravity and isKinematic off. When I click the mouse button, a sphere is spawned, but just floats there, unmoving. If I spawn another sphere overlapping the first, they both begin to move, floating off into space. This happens regardless of the number I assign as “speed”, whether it’s 100 or 1000000000.

Create another GameObject variable and have it equal the Instantiation. Then add force to it, not the prefab.