card deck null pointer exception

OK. So I've been working on a project and cannot seem to get on why the code is giving me the following error message:

"Exception in thread "main" java.lang.NullPointerException
at Card.getSuit(CardDeck.java:105)
at Card.toString(CardDeck.java:121)
at CardDeck.shuffle(CardDeck.java:33)
at CardDeck.<init>(CardDeck.java:16)
at CardDeck.main(CardDeck.java:70)"

import java.util.*;
import java.lang.NullPointerException;
import java.util.Random;

public class CardDeck {

	private Card[] deck = new Card[52];

	public CardDeck() {
		for (int i = 0; i < 13; i++) {
			deck[i] = new Card(i + 1, "DIAMOND");
			deck[i + 13] = new Card(i + 1, "SPADE");
			deck[i + 26] = new Card(i + 1, "CLUB");
			deck[i + 39] = new Card(i + 1, "HEARTS");
		}
		shuffle(0); //error
		getTopCard();
		cutDeck(0);

	}

	public void shuffle(int randomSeed) {
		Card[] temp = new Card[2];
		for (int i = 0; i < 20; i++) {
			Random b = new Random();
			Random a = new Random();
			temp[0] = deck[b.nextInt(51)];
			deck[b.nextInt(51)] = deck[a.nextInt(51)];
			deck[a.nextInt(51)] = temp[0];
		}
		System.out.print("Shuffled:");
		for (int i = 0; i < deck.length; i++) {
			System.out.print(deck[i].toString()); //error
			System.out.println("\n");
		}
	}

	private Card getTopCard() {
		return deck[0];
	}

	public void cutDeck(int location) {
		Random cut = new Random();
		int depth = cut.nextInt(51);
		Card[] top = new Card[depth];

		for (int i = 0; i < depth; i++) {
			top[i] = deck[i];
		}
		// copy all cards after and including the pivot to
		// the beginning of the array
		for (int i = 0; i < (deck.length - depth); i++) {
			deck[i]  = deck[depth + i];
		}
		// copy the cards from the temp array to the end of the array
		for (int i = 0; i < depth; i++) {
			deck[deck.length - depth + i] = top[i];
		}
		// return that it worked
		System.out.println("Cut with : ");
		System.out.println(depth);
		for (int i = 0; i < deck.length; i++) {
			System.out.print(deck[i].toString());
			System.out.println("\n");
		}

	}

	public static void main(String args[]) {
		CardDeck launch = new CardDeck();	//error
	}
}

class Card {
	private static enum Suits {
		SPADE, CLUB, DIAMOND, HEART
	};

	private int value; // default does not make sense!
	// public Suits suit;
	private Suits suit;

	public Card(int aValue, String aSuit) {
		value = aValue;

		aSuit = aSuit.toUpperCase();

		if (aSuit.compareTo("SPADE") == 0)
			suit = Suits.SPADE;
		if (aSuit.compareTo("CLUB") == 0)
			suit = Suits.CLUB;
		if (aSuit.compareTo("DIAMOND") == 0)
			suit = Suits.DIAMOND;
		if (aSuit.compareTo("HEART") == 0)
			suit = Suits.HEART;


	} // constructor

	public int getValue() {
		return value;
	}

	public String getSuit() {
		switch (this.suit) {   //error

		case SPADE:
			return "SPADE";
		case CLUB:
			return "CLUB";
		case DIAMOND:
			return "DIAMOND";
		case HEART:
			return "HEART";
		}

		return null;
	}

	public String toString() {
		return "<" + this.getValue() + " " + this.getSuit() + ">"; //error
	}
}

It works when I compile and run it however it throws the exception at the end. It does not give me any errors before I try to run it, it just states that exception. How can I get rid of it? Any help is greatly appreciated.