WWW.bytesDownloaded can't work, use LoadFromCacheOrDownload()

When I use WWW.LoadFromCacheOrDownload, Error Occurred！！！

code:

 ...
 WWW Tmp3W = WWW.LoadFromCacheOrDownload(DLPath, 1);
 
 while (true) {
 
     if (Tmp3W.isDone) {
         Debug.Log(Tmp3W.bytesDownloaded); // Error: WWWCached data can only be accessed using the assetBundle property!
 
         yield return 1;
         break;
     } else {
         Debug.Log(Tmp3W.bytesDownloaded); // Error: WWWCached data can only be accessed using the assetBundle property!
     }
 }
 ...

BUT, If I use WWW(url)，there is no problem. The main purpose is to use "WWW.bytesDownloaded" to get my download progress.