Custom class that contains prefabs as members

I'm trying to write my own class "BlockUnit". The class represents one "piece" of the game map, each piece should contain a variety of things such as pick ups that spawn as well as obstacles. Most importantly, it should contain a floor, which I have laid out as 3x3 planes attached to the same empty gameobject, which I named "Block" (I really should have named it Floor or something, but ah well). I want to have a block manager that deletes Blocks when appropriate and handles the list of blocks. However, my problem now is that I can't get the Block class to instantiate the Floor prefab.

This is my constructor:

     public BlockUnit() {
         unitQuaternion.Set (0, 0, 0, 1);
         baseBlock = (GameObject) Instantiate (baseBlock, Vector3.zero, unitQuaternion);
         baseBlock.gameObject.SetActive (false);        // turn off the game object so it's invisible until needed
     }

The game runs when I click play, but it gives me an error right here saying the prefab I am trying to instantiate is null. I for sure dragged the Block prefab into the baseBlock field in the editor, so I'm not sure what is going wrong here.