Reason for Null triangulation

Hello

I create a face from Geom_Surface using BRepBuilderAPI_Makeface, then I using the Triangulation function in the BRep_Tool package ( see code below ). What is the reason for getting a NULL triangulation ?

----------------- My code -------------------- void Surface::DrawTriangulations(Handle(Geom_Surface)& S ) {

Standard_Integer result(0);

TopoDS_Face F = BRepBuilderAPI_MakeFace(S);

TopLoc_Location L;

Handle (Poly_Triangulation) facing = BRep_Tool::Triangulation(F,L);

if (!facing.IsNull()) {

cout

result = result + facing->NbTriangles();

} else {

cout

} } -------------------------------------------