Problem with raycasting when checking neighbouring objects?

Hello, I am creating a game that looks like this: http://gyazo.com/366aedddefb336d7052dec5d35876368

And I have a method called VerifyPlacement which takes a tile, and checks the four surrounding tiles to see if it is just a tile, or if it has one of those colorful symbols on it(in other words, it gets the objects that are above, below, to the left and to the right of the tile).

I am trying to do this by raycasting above each of the surrounding tiles, like so:

         Vector2 checkHere = new Vector2 (c.x + s.x, c.y); //"c" is the collider, and "s" is the size of the collider. so in this case, it should cast a ray exactly one tile to the right.
         RaycastHit2D hit = Physics2D.Raycast(checkHere, Vector2.zero);
         Debug.Log (hit.transform.gameObject);

However, I get an error message on the debug.log which is:

NullReferenceException: Object reference not set to an instance of an object TileController.VerifyPlacement (UnityEngine.GameObject obj) (at Assets/Scripts/TileController.cs:41)

Is there a simple way to fix this problem and get my raycasting to work? Thanks for the help.