Inconsistency error C#
<p>I've got an issue with <code>protected SceneItem scene = null;</code> but I can't see why, the error is: </p>

<blockquote>
  <p>Inconsistent accessibility: field type 'AsteroidsFinal.Helpers.SceneItem' is less accessible than field 'AsteroidsFinal.Helpers.Screen.scene'`</p>
</blockquote>

<pre><code>using System;
using System.Collections.Generic;
using System.Text;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;

namespace AsteroidsFinal.Helpers
{

abstract public class Screen
{
    #region Variables

    protected SceneItem scene = null;

    protected Screen overlay;

    protected SpriteBatch batch = null;

    protected Game game = null;

    #endregion

    #region Properties

    public Game GameInstance
    {
        get { return game; }
    }

    public SpriteBatch Sprites
    {
        get { return batch; }
    }    

    #endregion

    public Screen(AsteroidGame game)
    {
        this.game = game;

        if (game != null)
        {
            IGraphicsDeviceService graphicsService = (IGraphicsDeviceService)game.Services.GetService(typeof(IGraphicsDeviceService));
            this.batch = new SpriteBatch(graphicsService.GraphicsDevice);
        }
    }

    public virtual GameState Update(TimeSpan time, TimeSpan elapsedTime)
    {
        scene.Update(time, elapsedTime);

        return (overlay == null) ? GameState.None : overlay.Update(time, elapsedTime);
    }

    public virtual void Render()
    {
        scene.Render();

        if (overlay != null)
            overlay.Render();
    }

    public virtual void Shutdown()
    {
        if (overlay != null)
            overlay.Shutdown();

        if (batch != null)
        {
            batch.Dispose();
            batch = null;
        }
    }

    public virtual void OnCreateDevice()
    {
        IGraphicsDeviceService graphicsService = (IGraphicsDeviceService)game.Services.GetService(typeof(IGraphicsDeviceService));
        batch = new SpriteBatch(graphicsService.GraphicsDevice);
    }
}

}
</code></pre>
