assetBundle.mainAsset exception

Hi

I am writting code which loads a .unity3d file and uses it's asset to create a GameObject. I am builiding it for Android platform. I created a Cube prefab and then I built the assetBundle following this tutorial: http://docs.unity3d.com/Documentation/Manual/BuildingAssetBundles.html. Here is my code:

 WWW www = new WWW ("file://sdcard/Cube.unity3d");
 yield return www;
 try {
    GameObject obj = Instantiate(www.assetBundle.mainAsset) as GameObject;
 }
 catch (Exception ex) {
    gameObject.guiText.text = "Exception: " + ex.Message;
 }

The path "file://sdcard/Cube.unity3d" works because www.error is null, and i've checked that an error message appears if I use a wrong path. The problem occurs when I try to instantiate the mainAsset. Here, a exception is produced with the message: "Object reference not set to an instance of an object". I have checked that the exception is launched when trying to read the property mainAsset of the AssetBundle. I have tried to use assetbundle.Load instead, but the result is the same. I also tried to download the .unity3d file from a server instead using a local path.

Any idea?

Thank you