Boo: Object reference not set to an instance of an object

Hey Guys, I tried to use multidimensional arrays in boo and it just threw this error:

 NullReferenceException: Object reference not set to an instance of an object
     GameScript.buildGrid () (at Assets/Scripts/GameScript.boo:28)
     GameScript.OnGUI () (at Assets/Scripts/GameScript.boo:40)

Here's my Code:

 import UnityEngine
 
 public class Card (System.Object):
     isFaceUp as bool  = false
     isMatched as bool = false
     public img = "robot"
 
 
 class GameScript (MonoBehaviour):
     cols as int = 4
     rows as int = 4
     totalCards as int = cols * rows
     matchesNeededToWin as int = totalCards * 0.5
     mathesMade as int = 0
     cardW as int = 100
     cardH as int = 100
     aCards as List
     aGrid as (Card, 2)
     aCardsFlipped as List
     playerCanClick as bool
     playerHasWon as bool = false
     
     def buildGrid():
         GUILayout.BeginVertical()
         for i in range(rows):
             GUILayout.BeginHorizontal()
             for j in range(cols):
                 card = aGrid[i,j]
                 if (GUILayout.Button(Resources.Load(card.img), GUILayout.Width(cardW))):
                     Debug.Log(card.img)
                     
     def Start ():
         playerCanClick = true
         for i in range(rows):
             for j in range(cols):
                 aGrid[i,j] = Card()
     
     def OnGUI ():
         GUILayout.BeginArea (Rect (0,0,Screen.width,Screen.height))
         //buildGrid()
         GUILayout.EndArea()
 

Thanks in advance!