NullReferenceException being thrown when I try to add a component to a dictionary

I have the following section of code from a MonoBehaviour class in a voxel engine that I'm currently working on right now:

 public void CreateChunk(Position chunkPosition)
 {
     GameObject newChunk = (GameObject)Instantiate(
         this.ChunkPrefab,
         new Vector3(chunkPosition.X, chunkPosition.Y, chunkPosition.Z),
         Quaternion.identity
     );

     Debug.Log("New Chunk", newChunk);

     Chunk newChunkComponent = newChunk.GetComponent<Chunk>();
     newChunkComponent.ChunkPosition = chunkPosition;
     newChunkComponent.ParentTerrainManager = this;

     this._ChunksInMemory.Add(chunkPosition, newChunkComponent);

     for(int x = 0; x < Chunk.GlobalChunkSize; x++)
     {
         for(int y = 0; y < Chunk.GlobalChunkSize; y++)
         {
             for(int z = 0; z < Chunk.GlobalChunkSize; z++)
             {
                 newChunkComponent.CreateBlock(new Position(x, y, z), new Stone());
             }
         }
     }
 }

The Chunk class is also a MonoBehaviour class as well. I'm trying to instantiate a chunk prefab and then get the Chunk component from the instantiated prefab. Unfortunately I get the following NullReferenceException:

  NullReferenceException: Object reference not set to an instance of an object

  Voxel.Engine.TerrainManager.CreateChunk (Position chunkPosition) (at Assets/Source/Engine/TerrainManager.cs:99)

  Voxel.Engine.TerrainManager.Start () (at Assets/Source/Engine/TerrainManager.cs:132)

This is line 99:

 this._ChunksInMemory.Add(chunkPosition, newChunkComponent);

The field this._ChunksInMemory is simply a Dictionary<Position, Chunk> containing all the currently loaded chunks.

I have done the following things so far:

• I have confirmed that the TerrainManager MonoBehaviour class (the one that contains the above section of code) does in fact have a chunk prefab assigned.

• I have confirmed that there is in fact a copy of the chunk prefab by looking at the hierarchy and calling Debug.Log("Chunk created", newChunk).

• I have confirmed that the copy of the chunk prefab has a Chunk component attached to it.

At this point, I am clueless as to why my code isn't working. Just from looking at the code, it should work fine, but it doesn't. Does anyone know why this is happening?