Object reference to set to instance on an instantiated object

I have a C# script where I am instantiating a prefab inside of a coroutine. When I attempt to access that prefab from another coroutine or anywhere else in the code, I am getting a null reference exception; however, I know that the object exists, as I can see it on-screen, and I can reference it later in the same coroutine that instantiates it. Here is the relevant code:

 private Vector3 originalPosition = new Vector3 (0, 0, 0);
 public GameObject masterPrefab;
 private GameObject instantiatedPrefab;
 
 void Start ()
 {
      instantiatedPrefab = null;
 }
 
 private IEnumerator SpawnPrefab ()
 {
      if (instantiatedPrefab == null)
      {
           instantiatedPrefab = (GameObject)Instantiate (masterPrefab, originalPosition, new Quaternion (0, 0, 0, 0)) as GameObject;
      }
 
      print ("Prefab created at " + instantiatedPrefab.transform.position.ToString ());
 }

If I call StartCoroutine (SpawnPrefab ()), the prefab is instantiated. I can see it on the screen and it behaves as expected. The print statement accesses the instantiated prefab's position and prints it correctly. However, if I attempt to acces the instantiated prefab anywhere else -- in Update, or another coroutine, for example -- I get the null reference exception. So the following will throw that error:

 private IEnumerator DoSomethingWithPrefab ()
 {
      print ("Do something with prefab located at " + instantiatedPrefab.transform.position.ToString ());
 }

As well as this:

 void Update ()
 {
      print ("Prefab position: " + instantiatedPrefab.transform.position.ToString ());
 }

I'm stumped. I figured if I successfully instantiated the object, I would be able to manipulate it within the script, but I'm clearly missing something.