Simple Multidimensional Associative Array question.

I'm creating and indexing a multidimensional associative array like so:

arrayOfFruits["apples"] = {}
arrayOfFruits["apples"]["oranges"] = {};

I make some assignments later on:

arrayOfFruits["fuji"]["mandarin"] = 1;

Later on in the code, I try to go:

var a = "redDelicious";
var b = "florida";

if (arrayOfFruits[a] **== 1)**
**{**
**// do something with fruits**
**}**
**```**
**<p>On the "if" line, I get a NullReferenceExeption:  Object ref not set to instance or object.</p>**
**<p>I've tried:</p>**
**```**
__if (arrayOfFruits[a] **!= null)**__
__**```**__
__**<p>no dice.  :(</p>**__
__**<p>I know this must be terribly simple.</p>**__

I initialized all possible pairs of items in the 2D array to 0. This seems to have solved the problems.