Reading geometry from Slab elements in IModels

Hello,

I'm trying to read geometry from slab elements like the one that can be found in test.i.dgn here, but I'm running into an issue. These slabs have geometry of type Group, but when I try to iterate over the elements in these groups and read each one like in the code snipped attached, the group->Members() call is always returning a null pointer. This code does work for other types of groups, but slabs seem to be a problem. 

I would appreciate any ideas or solutions anyone might have!

Thanks for your time,

Aidan

processGroupGeom(Group^ group)
{
    for each (IGeometry^ member in group->Members())
    {
        // Go through each child and process it
        processGeometry(member);
    }
}