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.