2d array: finding out if an item is null?!?

My inventory script is structured as follows: A 2d array of GameObjects

 var InvMain : GameObject[ , ] = new GameObject[12,4]; 

first integer describes the inventory slot (12 total) and the second int describes the items in that slot, so stacked items can be stored. So far, so good.

Now, to display my stored inventory items, I use the simple code

 function OnGUI()
 {
      for (var x = 0; x < InvMain.Length; x ++)
      {
           if(InvMain[x,0] != null)//always show the first item of a stack of items
           {
                 //icon positions and sizes are stored in seperate lists that are not important for this question
                 //the coordinates here are just a placeholder
                GUI.DrawTexture (Rect(100 + x * 20, 100 + x * 20, 20, 20), InvMain[x,0].GetComponent(EL_Item).Icon);
           }
      }
 }

The problem here is the if(InvMain[x,0] != null) part, if my 2d array's second dimension is only size 1 (GameObject[12,1]), then it all works fine. If it's bigger than 1 however, then I get an "ArgumentOutOfRangeException: Argument is out of range." error.

It seems as even though I specifically check if the item at position [x,1], [x,2] and so on are not null, it still tries to access it and draw icon graphics etc.

How does one check if an item at a specific positions inside a 2d array is null? Also, my 2d array doesn't show up in the inspector :(