Why can't I get the transform from an instantiated object?

I'm instantiating an object (a signpost) and then trying to use its transform position as a base for defining where a textmesh is spawned. Thusly:

 using UnityEngine;
 using System.Collections;
 
 public class spawntestscript : MonoBehaviour {
     
     public Transform objectToSpawn ; //the prefab we're spawning
     //public GameObject spawnedSign ; //just so we can see it in the inspector
     public Transform newTextMesh ; //To define the text mesh for the 3D sign
     
     
     
     // Use this for initialization
     void Start () {
     
         
         GameObject spawnedSign = Instantiate (objectToSpawn, transform.position, transform.rotation) as GameObject; //spawn the sign
         
         Vector3 startPosition = spawnedSign.transform.position; //get the sign's position and set it as a variable
         
         //Vector3 startPosition = spawnedSign.GetComponent<Transform>().position; //alternate method, same result
         
                 
         startPosition.x += 38; //move the start position
         
         startPosition.y -=30; 
                 
         //via the instantiate method
         //We need to establish a start point and then += it after each block
         
         GameObject text3d = Instantiate (newTextMesh, startPosition, Quaternion.Euler(0,0,0)) as GameObject;  //create a text mesh at the adjusted start position
         
         
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }

Yet what I'm getting is "NullReferenceException: Object reference not set to an instance of an object spawntestscript.Start () (at Assets/Scripts/spawntestscript.cs:18)"

I'm convinced it's something simple and fundamental, but I've been searching for solutions for hours now and I'm coming up blank. Help?