cannot get bytes in WWW

Hi everyone. I try to store assetbundle but it donot work

  WWW download;
  string url ="file://"+ Application.dataPath + "/assetBundle/images";
  Debug.Log(url);
  download = WWW.LoadFromCacheOrDownload(url, 0);

   yield return download;

     AssetBundle assetBundle = download.assetBundle;
     if (assetBundle != null)
     {
         // Alternatively you can also load an asset by name (assetBundle.Load("my asset name"))
         GameObject go = assetBundle.LoadAsset<GameObject>("myasset");

         if (go != null)
             Instantiate(go);
         else
             Debug.Log("Couldn't load resource");
     }

    if (string.IsNullOrEmpty(download.error))
   {
            string cachedAssetBundle = Application.dataPath + "/savedassetbundle.assetbundle";
            System.IO.FileStream cache = new System.IO.FileStream(cachedAssetBundle, System.IO.FileMode.Create);
         byte[] bytes = download.bytes;
     cache.Write(bytes, 0, download.bytes.Length);
     cache.Close();

   }

here is error: WWWCached data can only be accessed using the assetBundle property! UnityEngine.WWW:get_bytes()

I can Instantiate my image but i can't get download.bytes?