C# piškvorky - kde môže byť chyba?

PlayBoard.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using static Piškvorky.GamePiece;

namespace Piškvorky
{
    public partial class PlayBoard : UserControl
    {
        private int boardSize = 19;
        private int fieldSize = 20;
        private Color colorGrid = Color.Gray;
        private Color colorX = Color.Red;
        private Color colorO = Color.Green;
        private GamePiece[,] piecesOnBoard;

        public GamePiece[,] PiecesOnBoard
        {
            get
            {
                if (piecesOnBoard == null)
                    ClearBoard();
                return piecesOnBoard;
            }
        }

        private void ClearBoard()
        {
            piecesOnBoard = new GamePiece[boardSize, boardSize];
            for (int x = 0; x < boardSize; x++)
                 for (int y = 0; y < boardSize; y++)
                    piecesOnBoard[x, y] = Free;
        }

        public int BoardSize
        {
            get { return boardSize; }
            set
            {
                boardSize = value;
                Refresh();
            }
        }

        public int FieldSize
        {
            get { return fieldSize; }
            set
            {
                fieldSize = value;
                Refresh();
            }
        }

        public Color ColorGrid
        {
            get { return colorGrid; }
            set
            {
                penGrid = null;
                colorGrid = value;
                Refresh();
            }
        }
        #region Pens

        private Pen penGrid;
        public Pen PenGrid
        {
            get
            {
                if (penGrid == null)
                    penGrid = new Pen(colorGrid);
                return penGrid;
            } 
        }

        private Pen penX;
        public Pen PenX
        {
            get
            {
                if (penX == null)
                    penX = new Pen(colorX);
            return penX;
            }
        }

        private Pen penO;
        public Pen PenO
        {
            get
            {
                if (penO == null)
                    penO = new Pen(colorO);
                return penO;
            }
        }

        #endregion 

        public PlayBoard()
        {
            InitializeComponent();
        }

        private void PlayBoard_Paint(object sender, PaintEventArgs e)
        {
            DrawBoard(e.Graphics);
        }

        private void DrawBoard(Graphics g)
        {
            for (int i = 0; i <= boardSize; i++)
            {
                g.DrawLine(PenGrid, 0, i * fieldSize, boardSize * fieldSize, i * fieldSize);
                g.DrawLine(PenGrid, i * fieldSize, 0, i * fieldSize, boardSize * fieldSize);
            }
        }

        private void DrawPieces(Graphics g)
        {
            for (int x = 0; x < boardSize; x++)
                for (int y = 0; y < boardSize; y++)
                    DrawPiece(g, PiecesOnBoard[x,y], x, y);
        }

        private void DrawPiece(Graphics g, GamePiece piece, int x, int y)
        {
            if (!(x < boardSize && x >= 0 && y < boardSize && y >= 0))
                throw new Exception("Súradnice sú mimo hraciu plochu");
            if (piece == X)
            {
                g.DrawLine(PenX,
                    x * fieldSize +1,
                    y * fieldSize +1,
                    x * fieldSize + fieldSize -1,
                    y * fieldSize + fieldSize -1);
                g.DrawLine(PenX,
                    x * fieldSize + 1,
                    y * fieldSize + fieldSize - 1,
                    x * fieldSize + fieldSize - 1,
                    y * fieldSize + 1);
            }
            if (piece == O)
            {
                g.DrawEllipse(PenO,
                    x * fieldSize + 1,
                    y * fieldSize + 1,
                    fieldSize - 2,
                    fieldSize - 2);
            }
        }

        private void PlayBoard_MouseClick(object sender, MouseEventArgs e)
        {
            int x = e.X/fieldSize;
            int y = e.Y/fieldSize;
            if (!(x < boardSize && x >= 0 && y < boardSize && y >= 0))
                return;
            piecesOnBoard[x, y] = GamePiece.X;
            Refresh();
        }
    }
}

GamePiece.cs

namespace Piškvorky
{
    public enum GamePiece
    {
        Free,
        X,
        O,
    }
}

---

Vím není to dokončené ale podle toho čo tam zatím je by malo to ísť spustiť a v prípade klikania všade hádzať len X, no mne to v poho otvori ale po kliknuti na nejake poličko apka sekne a vyskočí mi tohle:

[img=http://s18.postimg.org/ulqnn1urt/Bez_n_zvu.png]Bez_n_zvu.png[/img]

Viem vlastne čo to piše, ale i tak by to malo fungovať ne?