Rollerball Tutorial: rb.AddForce(movement * speed); not working?

Well I tried to research this issue to see how common it is and I've found a lot of errors mentioned that I don't seem to have. That being said, I can't find any errors elsewhere and I'm wondering if a second pair of eyes might help?

I am using C#, and the problem seems to come from line 24 based on the error I receive. I tried a couple different things as well, such as changing the command to rb.velocity, but it would not work because it can apparently not be used as a method.

Error:

NullReferenceException: Object reference not set to an instance of an object PlayerControls.FixedUpdate () (at Assets/Scripts/PlayerControls.cs:24)

Code: using UnityEngine; using System.Collections;

 public class PlayerControls : MonoBehaviour
 {
     //Public or Private Declarations
     public float speed = 100.0f;
     private Rigidbody rb;
 
     //initialization of movement
     void start()
     {
         rb = GetComponent<Rigidbody>();
     }
 
     //application to each frame of movement
     void FixedUpdate()
     {
         float moveHorizontal = Input.GetAxis("Horizontal"); //x variable
         float moveVertical = Input.GetAxis("Vertical"); //z variable
 
         Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
         
         rb.AddForce(movement * speed);
     }//end FixedUpdate
 }//end Startusing UnityEngine;