Can a wire cut a cap out of a sphere?

Hello!
According to the modelling data user guide, it should be possible to create a face from a surface and a wire. The wire is supposed to bound the face. If the wire is not embedded in the surface, it will be projected onto the surface.

This is the theory. In practice, I have not been able to cut a cap out of a spherical surface with a circular wire. Can somebody help to solve the problem? The code below can replace the MakeBottle() function in the opencascade tutorial. But when the created cap is added to the displayed compound (in the line "aBuilder.Add (aRes, cap);"), the application crashes:

$ run.sh
QImage::setColor: Index out of bound 2
Segmentation fault

What is wrong?

Thanks
Arno

Here is the code snippet:

TopoDS_Shape
MakeBottle(const Standard_Real myWidth , const Standard_Real myHeight ,
const Standard_Real myThickness)
{
//Create the face of a sphere
BRepPrimAPI_MakeSphere MS(1.);
TopoDS_Face aSphere = BRepPrimAPI_MakeSphere(1.);
//Get the spherical surface
Handle_Geom_Surface surf(BRep_Tool::Surface(aSphere));

//Create a circular wire, which lies on the spherical surface
gp_Pnt aP(0.5,0,0);
gp_Vec aV=gp::DX();
gp_Ax2 anAxes(aP, -aV);
gp_Circ aCirc(anAxes,sqrt(0.75));
BRepBuilderAPI_MakeEdge aCircle(aCirc);
BRepBuilderAPI_MakeWire MW(aCircle.Edge());

//Create a spherical cap, bounded by the circular wire
BRepBuilderAPI_MakeFace MF(surf,MW.Wire());
TopoDS_Face cap=MF.Face();

//Building the resulting compound
TopoDS_Compound aRes;
BRep_Builder aBuilder;
aBuilder.MakeCompound (aRes);
aBuilder.Add (aRes, cap);

//if the previous line is not commented, the application crashes:
//$ run.sh
//QImage::setColor: Index out of bound 2
//Segmentation fault

aBuilder.Add (aRes, MW.Wire());
return aRes;
}