Windows Form code not executing?

Ok, I've got a weird one here. I'm trying to make a basic tile engine using a windows form, but some of my code is just...not happening. I'll post the chunks in question.

private void MapEditor_Load(object sender, EventArgs e)
    {
        LoadImageList();
        FixScrollBarScales();
        cboCodeValues.Items.Clear();
        cboCodeValues.Items.Add("ENEMY");
        cboCodeValues.Items.Add("CHEST");
        cboCodeValues.Items.Add("NPC");


        for (int x = 0; x < 100; x++)
            cboMapNumber.Items.Add(x.ToString().PadLeft(3, '0'));

        cboMapNumber.SelectedIndex = 0;
        TileMap.EditorMode = true;
        backgroundToolStripMenuItem.Checked = true;
    }

This should be called when the form loads, right? The code dives into LoadImageList(), which contains:

private void LoadImageList()
    {
        string filepath = Application.StartupPath + @"\Content\Textures\IndoorTileSet.png";
        Bitmap tileSheet = new Bitmap(filepath);
        int tilecount = 0;
        for(int y = 0; y < tileSheet.Height / TileMap.TileHeight; y++)
        {
            for(int x = 0; x < tileSheet.Width / TileMap.TileWidth; x++)
            {
                Bitmap newBitmap = tileSheet.Clone(new System.Drawing.Rectangle(x * TileMap.TileWidth, y * TileMap.TileHeight, 

                    TileMap.TileWidth, TileMap.TileHeight), System.Drawing.Imaging.PixelFormat.DontCare);
                imgListTiles.Images.Add(newBitmap);
                string itemName = "";
                if(tilecount == 0)
                    itemName = "Empty";
                if(tilecount == 1)
                    itemName = "Floor";
                listTiles.Items.Add(new ListViewItem(itemName, tilecount++));
            }
        }

    }

The bitmap loads correctly, but then the entire MapEditor_Load method just stops working. tileCount seems to be a local variable in the debugger, and its value is 0, but the debugger never executes the breakpoint on the line which it is assigned. I have absolutely no idea why it would do this, and it's driving me nuts. Any help? Oh, I put the bitmap load in a try/catch block just to see if it was handling an exception in a weird way, but I had no luck. It's not throwing an exception. I began having this problem immediately after replacing my IndoorTileSet with an updated version.