How do I write a step with GDT's ?

I have the following code two create a shape and adding a dimension measurement to it:

    Standard_CString fname("somefile.step");

    // create shape
    TopoDS_Shape box1 = BRepPrimAPI_MakeBox(100.0, 100.0, 50.0).Shape();  // Base box
    TopoDS_Shape box2 = BRepPrimAPI_MakeBox(gp_Pnt(100.0, 50.0, 0.0), 50.0, 100.0, 50.0).Shape(); // Vertical box
    TopoDS_Shape lShape = BRepAlgoAPI_Fuse(box1, box2).Shape();



    // Create document
    Handle(TDocStd_Application) app = new TDocStd_Application;
    BinXCAFDrivers::DefineFormat(app);
    Handle(TDocStd_Document) doc;
    app->NewDocument("BinXCAF", doc);
    if (doc.IsNull()) {
        cout << "Document is null" << endl;
        return 1;
    }
    TDF_Label rootLab = doc->Main();
    Handle(XCAFDoc_ShapeTool) shapeTool = XCAFDoc_DocumentTool::ShapeTool(doc->Main());
    //TDF_Label shapeLab = shapeTool->AddShape(lShape);
        //  the same???
    TDF_Label shapeLab = shapeTool->NewShape();
    shapeTool->SetShape(shapeLab, lShape);



    // create dimension
    Handle(XCAFDoc_DimTolTool) dimTolTool = XCAFDoc_DocumentTool::DimTolTool(doc->Main());
    TDF_Label dimLab = dimTolTool->AddDimension();
    Handle(XCAFDoc_Dimension) dim;
    dimLab.FindAttribute(XCAFDoc_Dimension::GetID(), dim);
    if (dim.IsNull()) {
        cout << "Dimension is null" << endl;
        return 1;
    }
    Handle(XCAFDimTolObjects_DimensionObject) dimObj = dim->GetObject();
    dimObj->SetType(XCAFDimTolObjects_DimensionType_Location_LinearDistance);
    dimObj->SetPoint(gp_Pnt(0.0, 0.0, 0.0));
    dimObj->SetPoint2(gp_Pnt(150, 150, 150));
    dimObj->SetValue(99.99);
    dim->SetObject(dimObj);




    // link dimension to shape
    dimTolTool->SetDimension(shapeLab, dimLab);



    // write file
    STEPCAFControl_Writer writer;
    if (!writer.Transfer(doc)) {
        cout << "Failed to transfer" << endl;
        return 1;
    }
    writer.Write(fname);

The problem however is that if I try to save the document, the pmi is not being written. What am I doing wrong? The opencascade guide doesn't explain it. Please help.