BRepOffsetAPI_Sewing

Hi to all,

I have a very simple question regarding the BRepOffsetAPI_Sewing class. Does anybody know why it doesn't working for a single face? Until now I haven't any problem constructing a TopoDS_Shell from any number of faces but adding only a single face it returns always a TopoDS_Face instead of a TopoDS_Shell. I use the Perform() and SewedShape().

Thanks in advance
Andras