Assetbundle for Windows Phone 8.1 Universal - Error: assetbundle.mainAsset is null

Hi,

      I'm in the final stage of porting for Windows 8.1 Universal Platform (Both Phone & Desktop). But, I got stuck in the assetbundle, which is working fine in ios and Android not working on Windows Phone 8.1

I used WP8 Player as BuildTarget for Windows Phone 8.1 and MetroPlayer for Windows Desktop.

Desktop build working fine. But, Windows Phone 8.1 having the following problem

 private WWW wwwAssetBundle;
 using (WWW download=WWW.LoadFromCacheOrDownload(downloadUrl,1))
 {
      wwwAssetBundle = download;
      yield return download;
      if (download.error == null)
      {
           AssetBundle bundle_temp = download.assetBundle;
            // Here I'm getting the 'download.assetBundle' as null
           GameObject GO = Instantiate (bundle_temp.mainAsset) as GameObject;
           bundle_temp.Unload(false);          
      }
 }

download link has the asset. But It still showing 'bundle _temp' as null.

Is that I have done correct??

Help me out!!!!