Enemy ai's won't spawn in scene / IndexOutOfRangeException: Array index is out of range. EnemiesSpawner.Spawn () (at Assets/EnemiesSpawner.cs:17)

Sometimes my enemy ai's will spawn sometimes they won't spawn in the scene. I have two spawn points in two different locations in my scene and I have two different enemy ai's . I just want to spawn both of them and set the max for 20 every 5 seconds of so. One spawn point will work and the other one will work but the enemy ai will just stand where they spawn at . Sometimes some both enemy ai's will work . I am always getting this error :

IndexOutOfRangeException: Array index is out of range. EnemiesSpawner.Spawn () (at Assets/EnemiesSpawner.cs:17)

 using UnityEngine;
 using System.Collections;
 
 public class EnemiesSpawner : MonoBehaviour {
 public GameObject[] enemy;
 public Transform [] spawnPoints;
 public float spawnTime = 5f;
 public Vector3 spawnValues;
     
     void Start () {
      InvokeRepeating("Spawn", spawnTime, spawnTime); //Calls the "Spawn" function every 10 seconds.
 
     }
     
 
     void Spawn () {
     int spawnPointIndex = Random.Range (0, spawnPoints.Length -1);Instantiate(enemy[10], spawnPoints[spawnPointIndex].position, spawnPoints[spawnPointIndex].rotation);
              Vector3 spawnPosition = new Vector3 (Random.Range (-spawnValues.x, spawnValues.x), 1, Random.Range (-spawnValues.z, -spawnValues.z) ) ; // <= here
     }
 }