Adding object to a list does not work on non-host client

I'm making a grid-based turn-based strategy game with multiple units. Right now you can spawn units by pressing a key, which works fine. On the host client, spawning a unit adds that unit to the 'units' list, so that it can be selected/moved/etc.

However, if you spawn a unit on a non-host client, it's not added to the 'units' list for some reason. So, it is spawned, but it's impossible to select or move. Instead it just gives "NullReferenceException: Object reference not set to an instance of an object."

The list of units is in the Game Manager script:

 public static GameManager instance;
 public List<Unit> units = new List<Unit>();

There's a Player script which handles spawning:

 [Command]
 public void CmdSpawn()
  {
         obj = (GameObject)Instantiate(GameManager.instance.InfantryPrefab, transform.position, Quaternion.identity);
 
         newInf = obj.GetComponent<Infantry>();
         newInf.gridPosition = new Vector2(transform.position.x + Mathf.Floor(GameManager.instance.mapSize / 2), -transform.position.z + Mathf.Floor(GameManager.instance.mapSize / 2));
   
         // spawn on the clients
         NetworkServer.Spawn(obj);
 
         // adding newInf to the list of units
         GameManager.instance.units.Add(newInf);
     }

 public virtual void Update () {
  if (Input.GetKeyDown(KeyCode.J) && isLocalPlayer)
     {  
         CmdSpawn();
     }
  }

Is there a way to make it so the spawned units are added to the 'units' list on the non-host client as well?