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;

This might seem patronizing, but did you change the move speed in the inspector? it starts at 0 by default and it took me forever to figure out what was going on when i did it.