Unity - Null Reference when instantiating new object

I'm trying to cycle through a 2D array of card objects and instantiate them in my Unity scene. However, I'm getting a Null reference exception when trying to instantiate them. Here is the code that instantiates the objects:

//Setting up initial board
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++){
            //Checked with debug.log, this should work Debug.Log (board[j, i].name);
            Debug.Log(board[j, i].name);
            temp = Instantiate(board[j,i]) as GameObject;
            CardScript cs = temp.GetComponent<CardScript>();
            objectBoard[j, i] = cs;
            //Setting locations of the cards
            objectBoard[j, i].transform.position = new Vector3(30 * j + 20, 50 * i +   70, 0);
        }
    }

The error occurs in the line 'CardScript cs = new.... I originally had the error in the temp = Instantiate... line, when the code was GameObject temp = Instantiate.... It fixed when I made temp a private GameObject variable in the code. I don't think I can do that with this one, because I need to have a reference to each individual object I'm instantiating.

Full Code:

public class MatchScript : MonoBehaviour {

public CardScript[] potentialCards;

private CardScript[,] board;
private CardScript[,] objectBoard;
private List<CardScript> entries;
private GameObject temp;


// Use this for initialization
void Start () {
    entries = new List<CardScript> ();
    objectBoard = new CardScript[4,3];
    board = new CardScript[4, 3];
    foreach (CardScript c in potentialCards)
        entries.Add (c);
    foreach (CardScript c in potentialCards)
        entries.Add (c);

    //Loading up the board
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++){
            int k = Random.Range(0, entries.Count);
            board[j, i] = entries[k];

            entries.RemoveAt(k);
        }
    }

    //Setting up initial board
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++){
            //Checked with debug.log, this should work Debug.Log (board[j, i].name);
            Debug.Log(board[j, i].name);
            temp = Instantiate(board[j,i]) as GameObject;
            CardScript cs = temp.GetComponent<CardScript>();
            objectBoard[j, i] = cs;
            //Setting locations of the cards
            objectBoard[j, i].transform.position = new Vector3(30 * j + 20, 50 * i + 70, 0);
        }
    }
}

// Update is called once per frame
void Update () {

}