Problem with GeomConvert::SurfaceToBSplineSurface

Hi,

I need to convert a TopoDS_Face to a BsplineSurface.
I tryed this code :

TopoDS_Face F =TopoDS::Face(ex.Current());
BRepBuilderAPI_NurbsConvert nurbs(F);
Handle(Geom_Surface) geom_Extrusion = BRepLib_FindSurface(nurbs).Surface();
Handle(Geom_BsplineSurface) geombspline_extrusion = GeomConvert::SurfaceToBSplineSurface(geom_extrusion) ;

I have this compilation error :
error C2065: 'Handle_Geom_BsplineSurface' : undeclared identifier

Can someone tell me why ?

thanks,