How to use AvatarBuilder.BuildGenericAvatar() ?

I am trying to create a custom Avatar and AvatarMask based on a series of objects that are nested GameObjects with rotations, local positions and so on.

[code]

     var avatar = AvatarBuilder.BuildGenericAvatar(tempHandcraftedPrefab, "");
     avatar.name = handcraftedPrefab.name;

     GameObject go = new GameObject();
     UnityEngine.Object o = PrefabUtility.CreatePrefab(bCurrentSheet.BlendedOutputFolder + "/" + handcraftedPrefab.name + "_avatar.prefab", go );
     AssetDatabase.AddObjectToAsset(avatar, o);
     PrefabUtility.ReplacePrefab(go, o);
     AssetDatabase.Refresh();

[/code]

The above code creates an Avatar file, but it doesn't show any bones when I open it inside of the AvatarMask editor window.

I don't know where to go from here. Unity pros - can you give any examples of where you successfully used BuildGenericAvatar?