IEnumerator does not exist

 using UnityEngine;
 using System.Collections;
 
 public class DoorLogic : MonoBehaviour {
 
     public float guiBoxWidth;
     public float guiBoxHeight;
 
     public Transform theDoor;
 
     private bool drawGUI = false;
     public bool isClosed = true;
 
     // Update is called once per frame
     void Update () 
     {
         if (drawGUI == true && Input.GetKeyDown (KeyCode.E)) 
         {
             ChangeDoorState();
         }
     }
 
 
     void OnTriggerEnter(Collider collider)
     {
         if (collider.tag == "Player") 
         {
             Debug.Log("Working");
             drawGUI = true;
         }
     }
 
     void OnTriggerExit(Collider collider)
     {
         if (collider.tag == "Player") 
         {
             Debug.Log("Exit Working");
             drawGUI = false;
         }
     }
 
     void OnGUI()
     {
         if (drawGUI == true) 
         {
             if (isClosed == true)
                 GUI.Box (new Rect ((Screen.width / 2) - 51, 10, guiBoxWidth, guiBoxHeight), "Press E to Open");
 
             else
                 GUI.Box (new Rect ((Screen.width / 2) - 51, 10, guiBoxWidth, guiBoxHeight), "Press E to Close");
         }
     }
 
     void ChangeDoorState()
     {
         if (isClosed == true) 
         {
             theDoor.animation.CrossFade ("Opening");
             isClosed = false;
         }
 
         StartCoroutine (doorDelay(2));
 
         theDoor.animation.CrossFade ("Close");
         isClosed = true;
     }

    //IEnumerator is in redfont.
     IEnumerator doorDelay(float delaySec)
     {
         yield return new WaitForSeconds(delaySec);
     }
 }
 

I get this error "The name 'IEnumerator' does not exist in the current context". and the IEnumerator is in redfont.