Roll a Ball Tutorial problem

I used the code given to me in the site, I’m using Unity 5.4.3 with this project and every time I check the code it says
MonoBehaviour has unknown format!:
Base Type: MonoBehaviour ByteSize:-1 MetaFlag;32768 (node index; o)

This is the code I;m using
using UnityEngine;
using System.Collections;

public class PlayerController : MonoBehaviour {

    public float speed;

    private Rigidbody rb;

    void Start ()
        rb = GetComponent<Rigidbody>();

    void FixedUpdate ()
        float moveHorizontal = Input.GetAxis ("Horizontal");
        float moveVertical = Input.GetAxis ("Vertical");

        Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);

        rb.AddForce (movement * speed);

I use the error checker in Visual Studio but it says there are 0 errors, 0 warnings, 0 messages.

Hmm Is the Script itself named “PlayerController”?

Does the object the script is attached to have a RigidBody?

The Script looks the same as the one Unity has supplied so it should work. Let me know if any of my suggestions work.