The variable has not been assigned (but it has)

Hello, I have a script that sets up the world to play in. It has several prefabs assigned to it, and they work flawlessly. However, when I added a new prefab to the script and assigned it in the inspector, I get an error:

 UnassignedReferenceException: The variable copper of 'GameManager' has not been assigned.
 You probably need to assign the copper variable of the GameManager script in the inspector.
 UnityEngine.Object.Internal_InstantiateSingle (UnityEngine.Object data, Vector3 pos, Quaternion rot) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/UnityEngineObject.cs:73)
 UnityEngine.Object.Instantiate (UnityEngine.Object original, Vector3 position, Quaternion rotation) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/UnityEngineObject.cs:83)
 GameManager.SpawnBlocks () (at Assets/Scripts/GameManager.cs:37)
 GameManager.Start () (at Assets/Scripts/GameManager.cs:22)

It says that it is not assigned, but it is.

Code below:

 using UnityEngine;
 using System.Collections;
 
 public class GameManager : MonoBehaviour {
     
     public GameObject player;
     public GameObject ground;
     public GameObject inventory;
     public GameObject copper;
     private GameCamera cam;
     public int iB = 1;
     public int DBx;
     public int DBy;
     public string serial = "-DEVELOPERS---ONLY-";
     public bool canPlay = false;
 
     
     // Use this for initialization
     void Start () {
         cam = GetComponent<GameCamera>();
         SpawnPlayer ();
         SpawnBlocks ();
     }
     
     private void SpawnPlayer() {
         cam.SetTarget ((Instantiate (player, new Vector3( 0, 3, 0), Quaternion.identity) as GameObject).transform);
     }
 
     private void SpawnBlocks() {
         DBx = -30;
         DBy = -128;
         while(iB <= 7680) {
             Vector3 position = new Vector3(DBx, DBy, 0);
             Instantiate (ground, position, Quaternion.identity);
             int randomNumber = Random.Range(1, 1000);
             if(randomNumber > 800 && randomNumber < 900) {
                 Instantiate (copper, position, Quaternion.identity);
             }
             DBy++;
             if(DBy == 1) {
                 DBx++;
                 DBy = -128;
             }
             iB++;
         }
     }
 }