A Strange Quesyion

Hi Guys,

I have the code like

TopoDS_Edge tmpEdge=TopoDS::Edge(all_edges->Value(y));
Handle(Geom2d_Curve) tmpCurve=BRep_Tool::CurveOnSurface(tmpEdge,aFace,FirstB,LastB);

It works fine sometimes; however, sometimes my tmpEdge has a value but not the tmpCurve. can anyone tell me why?
and How to fix it?

Thanks!