IndexOutOfRangeException for initial spawn of prefabs from an array

I've been following along with a tutorial on Infinite Runners found here:

https://unity3d.com/learn/tutorials/modules/beginner/live-training-archive/infinite-runner

I'm doing a slightly modified version for my own project. Everything seems to work fine, except the game gives me an IndexOutOfRangeException immediately after I press the play button. Afterwards, the Spawn function seems to have no problems. Here's my code:

 using UnityEngine;
 using System.Collections;
 
 public class SpawnScript : MonoBehaviour {
 
     public GameObject[] obj;
     public float spawnMin = 3f;
     public float spawnMax = 4f;
 
     // Use this for initialization
     void Start () {
         Spawn ();
     }
 
     void Spawn () {
         int random = Random.Range(0, obj.Length);
         Debug.Log("Random array number: " + random + " GetLength: " + obj.GetLength(0) + " Length: " + obj.Length);
         Instantiate(obj[random], transform.position, Quaternion.identity);
         Invoke ("Spawn", Random.Range(spawnMin, spawnMax));
     }
 }

Here is a screencap of the quad this script is attached to:

(The quad is a child of the Main Camera object)

As you can see I added a logging statement, here's the console output, including the logging statement and error:

Random array number: 1 GetLength: 2 Length: 2

UnityEngine.Debug:Log(Object)

SpawnScript:Spawn() (at Assets/Scripts/SpawnScript.cs:29)

SpawnScript:Start() (atAssets/Scripts/SpawnScript.cs:14)

Random array number: 0 GetLength: 0 Length: 0

UnityEngine.Debug:Log(Object) SpawnScript:Spawn() (at Assets/Scripts/SpawnScript.cs:29)

SpawnScript:Start() (at Assets/Scripts/SpawnScript.cs:14)

IndexOutOfRangeException: Array index is out of range.

SpawnScript.Spawn () (at Assets/Scripts/SpawnScript.cs:30)

SpawnScript.Start () (at Assets/Scripts/SpawnScript.cs:14)

Random array number: 1 GetLength: 2 Length: 2

UnityEngine.Debug:Log(Object) SpawnScript:Spawn() (at Assets/Scripts/SpawnScript.cs:29)

Strangely enough, it seems that Start function calls Spawn() twice. The first time it shows the correct length of the array, but the second it shows an empty array. Any idea why this is happening?