How to get AssetBundleManifest from AssetBundle in Unity2018.

     using (WWW www = WWW.LoadFromCacheOrDownload("file://D:/TestUnity/AssetBundle/unity3d", version))
     {
         yield return www;
         if (www.error != null)
             throw new Exception("WWW download had an error:" + www.error);

         AssetBundle assetBundle = www.assetBundle;
         Debug.Log(assetBundle);
         AssetBundleManifest manifest =
             assetBundle.LoadAsset<AssetBundleManifest>("AssetBundleManifest");
         Debug.Log(manifest);
     }

Debug is

unity3d (UnityEngine.AssetBundle)

Null

Why not? I did Unity Manual(https://docs.unity3d.com/Manual/AssetBundles-Native.html )