edgeTriangulation isNul after connecting shapes coming from IGES file.

Hello.

After importing some IGES files and connecting included shapes, I get sometimes edgeTriangulation.isNull() on some or all edges.

Does anyone knows why ?
Thank you for help.
Jean-Yves

This an abstract of my code :

I first import IGES file:
-------------------------
IGESControl_Reader reader;
Standard_Integer readStatus = reader.ReadFile(filename);

switch (readStatus)
{
case IFSelect_RetVoid:
throw WException("Load IGES empty.");
case IFSelect_RetError:
throw WException("Load IGES error.");
case IFSelect_RetFail:
throw WException("Load IGES fail.");
case IFSelect_RetStop:
throw WException("Load IGES stop.");
default:
case IFSelect_RetDone:
// Load IGES readStatus=IFSelect_RetDone"
break;
}

// Tranlate IGES -> OCC.
reader.TransferRoots();

TopoDS_Shape initialShape = reader.OneShape();

Then, I sew the shape:
----------------------
if (sewing)
{
BRepOffsetAPI_Sewing sewedObj(sewingDistance);
sewedObj.Add(initialShape);
sewedObj.Perform();

if (!sewedObj.SewedShape().IsNull())
shape = sewedObj.SewedShape();
else
shape = initialShape;
}
else
shape = initialShape;

Then, I triangulate:
--------------------
BRepTools::Clean(shape);

BRepMesh_FastDiscret *TheDiscret = new BRepMesh_FastDiscret(
deflection, //const Standard_Real defle,
shape, //const TopoDS_Shape &shape,
bbox, //const Bnd_Box &B,
angle, //const Standard_Real angle,
Standard_True, //const Standard_Boolean withShare,
Standard_True, //const Standard_Boolean inshape,
Standard_False, //const Standard_Boolean relative,
Standard_False //const Standard_Boolean shapetrigu
);

for (exp02.Init(aShape, TopAbs_FACE); exp02.More(); exp02.Next())
{
TopoDS_Face face = TopoDS::Face(exp02.Current());

if (!faceTriangulation.IsNull())
{
TopExp_Explorer exp03;
for (exp03.Init(face, TopAbs_EDGE); exp03.More(); exp03.Next())
{
TopoDS_Edge edge = TopoDS::Edge(exp03.Current().Composed(face.Orientation()));

// Edge triangulation
Handle_Poly_PolygonOnTriangulation edgeTriangulation = BRep_Tool::PolygonOnTriangulation(edge, faceTriangulation, edge.Location());
if (edgeTriangulation.IsNull())
cout

}
}
}