sets/Scripts/GameController.cs(15,10): error CS1624: The body of `GameController.Start()' cannot be an iterator block because `void' is not an iterator interface type

 using UnityEngine;
 using System.Collections;
 
 public class GameController : MonoBehaviour
 {
     public Transform enemy;
     public float timeBeforeSpawning = 1.5f;
     public float timeBetweenEnemies = .25f;
     public float timeBeforeWaves = 2.0f;
 
     public int enemiesPerWave = 10;
     private int currentNumberOfEnemies = 0;
 
     // Use this for initialization
     void Start()
     {
         StartCoroutine(SpawnEnemies());
         IEnumerator SpawnEnemies;
         { 
             yield return new WaitForSeconds(timeBeforeSpawning);
             while (true)
             {
                 if (currentNumberOfEnemies <= 0)
                 {
                     float randDirection;
                     float randDistance;
                     for (int i = 0; i < enemiesPerWave; i++)
                     {
                         randDistance = Random.Range(10, 25);
                         randDirection = Random.Range(0, 360);
                         float posX = this.transform.position.x + (Mathf.
                             Cos((randDirection) * Mathf.Deg2Rad) * randDistance);
                         float posY = this.transform.position.y + (Mathf.
                             Sin((randDirection) * Mathf.Deg2Rad) * randDistance);
                         Instantiate(enemy, new Vector3(posX, posY, 0), this.transform.rotation);
                         currentNumberOfEnemies++;
                         yield return new WaitForSeconds(timeBetweenEnemies);
 
                     }
                 }
             }
 
         }
 
     }
 
     // Update is called once per frame
     void Update()
     {
 
     }
 
 
     public void KilledEnemy()
     {
         currentNumberOfEnemies--;
     }
 }