IS this situation a bug of TopoDS_Shape.IsNull() ?

Hey Bros, I meet a confusing situation today.
I build two faces: aFace and bFace, and the aFace can cover bFace.
Then I use the BRepAlgoAPI_Cut get a TopoDS_Shape which is named bCuta,
and as expected the bCuta has nothing, I had output it as step file to verify it.

And what make me confused is, when I call bCuta.IsNull(), the result is false.
I hace no idea why it's false???

There is my code:
gp_Pnt p1(0, 0, 0);

gp_Pnt p2(10, 0, 0);

gp_Pnt p3(10, 10, 0);

gp_Pnt p4(0, 10, 0);

TopoDS_Edge e1 = BRepBuilderAPI_MakeEdge(p1, p2);

TopoDS_Edge e2 = BRepBuilderAPI_MakeEdge(p2, p3);

TopoDS_Edge e3 = BRepBuilderAPI_MakeEdge(p3, p4);

TopoDS_Edge e4 = BRepBuilderAPI_MakeEdge(p4, p1);

TopoDS_Wire aWire = BRepBuilderAPI_MakeWire(e1, e2, e3, e4);

TopoDS_Face aFace = BRepBuilderAPI_MakeFace(aWire);

gp_Pnt p5(10, 5, 0);

gp_Pnt p6(0, 5, 0);

TopoDS_Edge e01 = BRepBuilderAPI_MakeEdge(p1, p2);

TopoDS_Edge e02 = BRepBuilderAPI_MakeEdge(p2, p5);

TopoDS_Edge e03 = BRepBuilderAPI_MakeEdge(p5, p6);

TopoDS_Edge e04 = BRepBuilderAPI_MakeEdge(p6, p1);

TopoDS_Wire bWire = BRepBuilderAPI_MakeWire(e01, e02, e03, e04);

TopoDS_Face bFace = BRepBuilderAPI_MakeFace(bWire);

TopoDS_Shape bCuta = BRepAlgoAPI_Cut(bFace, aFace);

if (bCuta.IsNull()) std::cout << "bCuta is null!" << std::endl;