Turning NPC Code not functioning

When I call this coroutine, the NPC fully turns instantly and then the logs keep continuing as he faces me. I have similar code used for flashing an object so I'm not sure why this isn't working.

     IEnumerator TurnNPC(){
         float t = 0;
         Vector3 introVector = m_NPC.transform.position;
         Vector3 endVector = new Vector3(GameObject.Find ("FPSController").transform.position.x, m_NPC.transform.position.y, GameObject.Find ("FPSController").transform.position.z);
 
         while(t<1){
             t+= Time.smoothDeltaTime / turnSpeed;//turnSpeed currently set to 1
             Debug.Log ("turn");
             m_NPC.transform.LookAt (Vector3.Slerp (introVector, endVector,t));
 
             yield return null;
         }
     }