NRE when accessing another script's variable?

I'm not quite sure why this isn't working, if anybody can help me out I would be very grateful. This is the script I'm using:

 using UnityEngine;
 using System.Collections;
 using AC;
 
 public class ToggleCursorManager : MonoBehaviour {
 
     void Start(){
         PlayerInput playerInput = GameObject.FindGameObjectWithTag("PersistentEngine").GetComponent <PlayerInput>();
         playerInput.cursorIsLocked = true;
     }
 }

Not to complicated, right? The error is on line 9:

NullReferenceException: Object reference not set to an instance of an object ToggleCursorManager.Start () (at Assets/Scripts/ToggleCursorManager.cs:9)

And just for good measure, here is the line defining the variable I'm trying to call (in another script):

 [HideInInspector] public bool cursorIsLocked = false;

I'm completely confused as to what's going on with this. How do I fix this NRE? Thank you in advance for any help!