Concat() for Lists

I've got a list called GhostObjects:

 private var GhostObjects : List.<GameObject> = List.<GameObject>();

There are either two types of ghosts in the scene, big ones and small ones:

 function Start () {
     GhostObjects = GameObject.FindGameObjectsWithTag("BigGhost").Concat(GameObject.FindGameObjectsWithTag("SmallGhost")).ToArray();
 }

The error I'm getting, is that it cannot convert:

 UnityEngine.GameObject[] to System.Collections.Generic.List<UnityEngine.GameObject>

How do I resolve this error?

Thanks!