gameobject null

The following script always return null.here i try to get gameobject from asset bundle…

UnityWebRequest www = UnityWebRequestAssetBundle.GetAssetBundle(@"file:\\xxxxxxx\Name.unity3d"); 
        yield return www.SendWebRequest();
        if (www.isNetworkError || www.isHttpError)
        {
            Debug.Log(www.error);
        }
        else
        {
            var bundle = DownloadHandlerAssetBundle.GetContent(www);
                            // here i can get asset bundle next line return null 
            GameObject cube = bundle.LoadAsset< GameObject>("Cube");
            yield return cube;
               Instantiate(cube, new Vector3(0, 0, 0f), Quaternion.identity);
        }

unless your asset(prefab) does not have a extention its probable “Cube.prefab” instead of just Cube.