Mask not working

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace MaskingTest
{
    /// <summary>
    /// This is the main type for your game.
    /// </summary>
    public class Game1 : Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;

        Texture2D mask, texture;

        public Game1()
        {
            graphics = new GraphicsDeviceManager(this);
            graphics.PreferredDepthStencilFormat = DepthFormat.Depth24Stencil8;

            Content.RootDirectory = "Content";
        }

        protected override void Initialize()
        {

            base.Initialize();
        }


        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);

            mask = Content.Load<Texture2D>("mask");
            texture = Content.Load<Texture2D>("texture");
        }

        protected override void UnloadContent()
        {

        }

        protected override void Update(GameTime gameTime)
        {

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            DrawMask();

            base.Draw(gameTime);
        }

        private void DrawMask()
        {
            GraphicsDevice.Clear(ClearOptions.Target | ClearOptions.Stencil, Color.Transparent, 0, 0);

            var m = Matrix.CreateOrthographicOffCenter(0,
                graphics.GraphicsDevice.PresentationParameters.BackBufferWidth,
                graphics.GraphicsDevice.PresentationParameters.BackBufferHeight,
                0, 0, 1
            );

            var a = new AlphaTestEffect(graphics.GraphicsDevice)
            {
                Projection = m
            };

            var s1 = new DepthStencilState
            {
                StencilEnable = true,
                StencilFunction = CompareFunction.Always,
                StencilPass = StencilOperation.Replace,
                ReferenceStencil = 1,
                DepthBufferEnable = false,
            };

            var s2 = new DepthStencilState
            {
                StencilEnable = true,
                StencilFunction = CompareFunction.LessEqual,
                StencilPass = StencilOperation.Keep,
                ReferenceStencil = 1,
                DepthBufferEnable = false,
            };

            spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s1, null, a);
            spriteBatch.Draw(mask, Vector2.Zero, Color.White); //The mask
            spriteBatch.End();

            spriteBatch.Begin(SpriteSortMode.Immediate, null, null, s2, null, a);
            spriteBatch.Draw(texture, Vector2.Zero, Color.White); //The background
            spriteBatch.End();
        }
    }
}