Passing Transform into Coroutine

         public Object prefab;
     
         void Start () {
         Transform obj = (Transform)Instantiate(prefab);
             StartCoroutine(routine(obj));
         }
     
         IEnumerator routine(Transform obj){
             print(obj);
         }

I get 'Null'

Since you can't use 'ref' in a coroutine, how could I pass the exact transform object into 'routine()'?