Export Solid to STEP file not working

Hello,

I'm trying to export a solid (simple box) into a STEP file. The problem is, that the exported geometry is not a solid. You can see the problem in the attached Screenshot (STEP_BlockReference.png).

Below is an excerpt of the code which exports the solid into the STEP file.

TopoDS_Solid CreateTestSolid();

BOOST_AUTO_TEST_CASE(ExportSolidToStep)
{
	TopoDS_Solid solid = CreateTestSolid();

	shared_ptr<string> step_target_file(new string(OUTPUT_BASE_PATH + "test.stp"));
	STEPControl_Writer step_writer = STEPControl_Writer();
	
	IFSelect_ReturnStatus result = step_writer.Transfer(solid, STEPControl_ManifoldSolidBrep, Standard_True);
	BOOST_TEST(result == IFSelect_RetDone);

	result = step_writer.Write(step_target_file->c_str());
	BOOST_TEST(result == IFSelect_RetDone);
}

TopoDS_Solid CreateTestSolid() {
	gp_Pnt a = gp_Pnt(1, 1, 1);
	gp_Pnt b = gp_Pnt(8, 1, 1);
	gp_Pnt c = gp_Pnt(8, 1, 4);
	gp_Pnt d = gp_Pnt(1, 1, 4);
	gp_Pnt e = gp_Pnt(1, 4, 1);
	gp_Pnt f = gp_Pnt(1, 4, 4);
	gp_Pnt g = gp_Pnt(8, 4, 4);
	gp_Pnt h = gp_Pnt(8, 4, 1);

	TopoDS_Edge a_b = BRepBuilderAPI_MakeEdge(a, b).Edge();
	TopoDS_Edge b_c = BRepBuilderAPI_MakeEdge(b, c).Edge();
	TopoDS_Edge c_d = BRepBuilderAPI_MakeEdge(c, d).Edge();
	TopoDS_Edge d_a = BRepBuilderAPI_MakeEdge(d, a).Edge();
	BRepBuilderAPI_MakeFace A = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(a_b, b_c, c_d, d_a).Wire());

	TopoDS_Edge b_h = BRepBuilderAPI_MakeEdge(b, h).Edge();
	TopoDS_Edge h_g = BRepBuilderAPI_MakeEdge(h, g).Edge();
	TopoDS_Edge g_c = BRepBuilderAPI_MakeEdge(g, c).Edge();
	TopoDS_Edge c_b = BRepBuilderAPI_MakeEdge(c, b).Edge();
	BRepBuilderAPI_MakeFace B = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(b_h, h_g, g_c, c_b).Wire());

	TopoDS_Edge h_e = BRepBuilderAPI_MakeEdge(h, e).Edge();
	TopoDS_Edge e_f = BRepBuilderAPI_MakeEdge(e, f).Edge();
	TopoDS_Edge f_g = BRepBuilderAPI_MakeEdge(f, g).Edge();
	TopoDS_Edge g_h = BRepBuilderAPI_MakeEdge(g, h).Edge();
	BRepBuilderAPI_MakeFace C = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(h_e, e_f, f_g, g_h).Wire());

	TopoDS_Edge e_a = BRepBuilderAPI_MakeEdge(e, a).Edge();
	TopoDS_Edge a_d = BRepBuilderAPI_MakeEdge(a, d).Edge();
	TopoDS_Edge d_f = BRepBuilderAPI_MakeEdge(d, f).Edge();
	TopoDS_Edge f_e = BRepBuilderAPI_MakeEdge(f, e).Edge();
	BRepBuilderAPI_MakeFace D = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(e_a, a_d, d_f, f_e).Wire());

	TopoDS_Edge e_h = BRepBuilderAPI_MakeEdge(e, h).Edge();
	TopoDS_Edge h_b = BRepBuilderAPI_MakeEdge(h, b).Edge();
	TopoDS_Edge b_a = BRepBuilderAPI_MakeEdge(b, a).Edge();
	TopoDS_Edge a_e = BRepBuilderAPI_MakeEdge(a, e).Edge();
	BRepBuilderAPI_MakeFace E = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(e_h, h_b, b_a, a_e).Wire());

	TopoDS_Edge d_c = BRepBuilderAPI_MakeEdge(d, c).Edge();
	TopoDS_Edge c_g = BRepBuilderAPI_MakeEdge(c, g).Edge();
	TopoDS_Edge g_f = BRepBuilderAPI_MakeEdge(g, f).Edge();
	TopoDS_Edge f_d = BRepBuilderAPI_MakeEdge(f, d).Edge();
	BRepBuilderAPI_MakeFace F = BRepBuilderAPI_MakeFace(BRepBuilderAPI_MakeWire(d_c, c_g, g_f, f_d).Wire());

	BRepBuilderAPI_Sewing sewing = BRepBuilderAPI_Sewing();
	sewing.Init();
	sewing.Add(A);
	sewing.Add(B);
	sewing.Add(C);
	sewing.Add(D);
	sewing.Add(E);
	sewing.Add(F);
	sewing.Perform();

	const TopoDS_Shape& shape = sewing.SewedShape();
	BOOST_TEST(shape.ShapeType() == TopAbs_SHELL);

	const TopoDS_Shell& shell = (const TopoDS_Shell&)shape;
	TopoDS_Solid solid = BRepBuilderAPI_MakeSolid(shell).Solid();
	return solid;
}

If i export the solid into an IGES file it works correctly (See attached Screenshot "IGES_Solid.png").

The code below exports the solid into the IGES file:

BOOST_AUTO_TEST_CASE(ExportSolidToIges)
{
	TopoDS_Solid solid = CreateTestSolid();

	shared_ptr<string> iges_target_file(new string(OUTPUT_BASE_PATH + "test.iges"));
	IGESControl_Controller::Init();
	IGESControl_Writer iges_writer("mm", 1);
	iges_writer.AddShape(solid);
	iges_writer.Write(iges_target_file->c_str());
}

It would be great if someone can tell me what I'm doing wrong in the example above.

Thank you.