help! about Geom_Surface class

gp_Ax3 ax;
Handle(Geom_SphericalSurface) geom_sphere;
geom_sphere=new Geom_SphericalSurface(ax,1.0);
TopoDS_Shell shell=BRepBuilderAPI_MakeShell(geom_sphere);

This works fine! But the followings can not work

gp_Ax3 ax;
Handle(Geom_ConicalSurface) geom;
geom=new Geom_ConicalSurface(ax,0.5,1.0);
TopoDS_Shell shell=BRepBuilderAPI_MakeShell(geom);

It will always give me error "Segment fault!", I think for last line
"TopoDS_Shell shell=BRepBuilderAPI_MakeShell(geom);"