Re-load prefabs from assetbundles got null components

Hi, I find a strange behaviour of Unity when re-loading prefab from assetbundle. Say I have a assetbundle A, which contains a prefab with a BoxCollider and a CharacterController attached. Then I execute following codes:

 GameObject m_prefab = A.Load ("prefab") as GameObject;
 Collider collider = m_prefab.GetComponent<BoxCollider> ();
 Debug.Log (collider);
 CharacterController cc = m_prefab.GetComponent<CharacterController> ();
 Debug.Log (cc);
 GameObject.DestroyImmediate (m_prefab, true);

Then I repeat above codes in somewhere else, but it turns out that BoxCollider and CharacterController are all missing in the re-loaded prefab, while the first loaded one is correct.

WHY?