InvalidCastException: Cannot cast from source type to destination type - can't seem to instantiate objects into an array!

The following returns an Invalid Cast Exception. I'm trying to instantiate a load of tiles into a 2d array so I can refer to them easily. The line

 tileTable[activeX,activeY] = (Tile)Instantiate(Tile, new Vector3(activeX,activeY,0), Quaternion.identity);

is where the error is reported.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class TileEngine : MonoBehaviour {
 
     public Vector2 mapSize;
     public GameObject Tile;
 
     Tile[,] tileTable;
     Vector2 chunkCodes;
 
     // Use this for initialization
     void Start () {
         tileTable = new Tile[(int)mapSize.x,(int)mapSize.y];
         chunkCodes = new Vector2((int)mapSize.x/10,(int)mapSize.y/10);
         Debug.Log (chunkCodes);
         Debug.Log (tileTable.Length);
         BuildMap();
     }
 
     void BuildMap(){
         Vector2 chunkCode = new Vector2(0,0);
         for (int x = 0; x <= chunkCodes.x; x++){
             chunkCode.y = 0;
             for (int y = 0; y <= chunkCodes.y; y++){
                 MakeChunk(chunkCode);
                 chunkCode.y = chunkCode.y + 10;
             }
             chunkCode.x = chunkCode.x + 10;
         }
     }
 
     void MakeChunk (Vector2 activeChunk){
         Debug.Log (activeChunk);
         int activeX;
         int activeY;
         for (int x = 0; x < 10; x++){
             activeX = x + (int)activeChunk.x;
             for (int y = 0; y < 10; y++){
                 activeY = y + (int)activeChunk.y;
                 tileTable[activeX,activeY] = (Tile)Instantiate(Tile, new Vector3(activeX,activeY,0), Quaternion.identity);
                 tileTable[activeX,activeY].chunkCode = activeChunk;
             }
         } 
     }
 
 }

Any help is much appreciated.

Thanks