IndexOutOfRangeException: Array index is out of range.

Please help me. How to edit this error I use Unity version 5.1.1f

IndexOutOfRangeException: Array index is out of range. EnemySpawner.SpawnEnemy (UnityEngine.GameObject enemy) (at Assets/scripts/enemy/EnemySpawner.cs:81) EnemySpawner.SpawnNext () (at Assets/scripts/enemy/EnemySpawner.cs:69) EnemySpawner.Update () (at Assets/scripts/enemy/EnemySpawner.cs:52)

EnemySpawner Script:

 /*
  * By Floris de Haan @ June 2015
  * Free to use for all purposes.
  */
 
 using UnityEngine;
 using System.Collections.Generic;
 
 /// <summary>
 /// Multifunctional enemy spawner class.
 /// </summary>
 public class EnemySpawner : MonoBehaviour
 {
     #region Vars
 
     [SerializeField]
     private EnemySpawnInfo[] _enemySpawnInfo;
     [SerializeField]
     private Transform _enemyParent;
     [SerializeField]
     private Transform[] _enemySpawnPositions;
 
     private List<GameObject> _enemies;
 
     [SerializeField]
     private float _spawnInterval = 1f;
     private float _spawnIntervalDecreasement = 0.01f;
     private float _spawnIntervalMin = 0.2f;
     private float _spawnTimer = 0f;
 
     [SerializeField]
     private int _maxEnemies = 20;
 
     #endregion
 
     #region Methods
 
     private void Start()
     {
         _enemies = new List<GameObject>();
     }
 
     private void Update()
     {
         if (_spawnTimer < _spawnInterval)
         {
             _spawnTimer += Time.deltaTime;
 
             if (_enemies.Count < _maxEnemies && _spawnTimer > _spawnInterval)
             {
                 _spawnTimer -= _spawnInterval;
                 SpawnNext();
                 if (_spawnInterval > _spawnIntervalMin) _spawnInterval -= _spawnIntervalDecreasement;
             }
         }
     }
 
     private void SpawnNext()
     {
         // Random percentage
         int rnd = Random.Range(0, 99);
         
         foreach (EnemySpawnInfo item in _enemySpawnInfo)
         {
             // Check if random is in chance
             if (rnd < item.chance)
             {
                 print("Spawn item: " + item.Enemy.name);
                 SpawnEnemy(item.Enemy);
                 break;
             }
 
             // Fix chance for next item
             rnd -= (int)item.chance;
         }
     }
 
     private void SpawnEnemy(GameObject enemy)
     {
         // Random spawn pos
         Transform rndSpawn = _enemySpawnPositions[Random.Range(0, _enemySpawnPositions.Length)];
 
         // Spawn
         GameObject newEnemy = (GameObject)Instantiate(enemy, rndSpawn.position, rndSpawn.rotation);
         if (_enemyParent != null) newEnemy.transform.parent = _enemyParent;
         _enemies.Add(newEnemy);
 
         // Wait untill dead
         EnemyHandler enemyHandler = newEnemy.GetComponent<EnemyHandler>();
         enemyHandler.GetDamageEvent += () => {
             RemoveEnemy(newEnemy);
         };
     }
 
     private void RemoveEnemy(GameObject enemy)
     {
         _enemies.Remove(enemy);
 
         // Reset timer to spawn next if already stopped
         if (_spawnTimer >= _spawnInterval) _spawnTimer = 0f;
     }
 
     #endregion
 
     #region Properties
 
     #endregion
 }
 
 [System.Serializable]
 public struct EnemySpawnInfo
 {
     public GameObject Enemy; // Prefab
     public short chance; // (0 to 99) %
 }