NullReference Exception for instantiate

Im running this code. It outputs 0 (meaning a gameobject called floor does exist)

Then outputs an error:

NullReferenceException: Object reference not set to an instance of an object TerrainGenerator.Start () (at Assets/TerrainGenerator.cs:13)

I have been attempting to get this to work for over an hour now. Could someone explain what is going on?

 using UnityEngine;
 using System.Collections;
 
 
 public class TerrainGenerator : MonoBehaviour {
     private GameObject cube;
 
     void Start () {
         Random.seed = 42;
         for (int i = 0; i < 20; i = i + 1){
             GameObject cube = Instantiate(transform.Find("dirt"),new Vector3(-30,0, i), Quaternion.identity) as GameObject;
             Debug.Log (GameObject.Find("floor").transform.position.z);
             cube.transform.parent = GameObject.Find("floor").transform;
         }
     }
 }