Error writing STEP file

Hello,

I believe I have found a bug in OpenCascade 6.2. I am able to load a large STEP model into memory without errors, but when I try to write it back out to disk, the following happens:

This is my code:

STEPControl_Writer writer;
writer.Model( Standard_True );
...
TopoDS_Shape shape = cascadeShape->getShapeImpl();

writer.Transfer( shape, STEPControl_AsIs );

And here is where the crash occurs deep inside of the OpenCascade library:

void TopoDSToStep_MakeStepFace::Init(const TopoDS_Face& aFace,
TopoDSToStep_Tool& aTool,
const Handle(Transfer_FinderProcess)& FP)
{

...
for (;Ex.More(); Ex.Next()) {
TopoDS_Edge E = TopoDS::Edge(Ex.Current());
Standard_Real cf, cl;
Handle(Geom2d_Curve) C2d = BRep_Tool::CurveOnSurface(E, ForwardFace, cf, cl);

//CA = BRepAdaptor_Curve(E, ForwardFace);
//GeomAbs_CurveType typCOnS = CA.CurveOnSurface().GetCurve().GetType();

//if (typCOnS == GeomAbs_Line && BRep_Tool::Degenerated(E) ) {
if ( //:abv 26Jan00, CAX-IF TRJ3: C2d->IsKind(STANDARD_TYPE(Geom2d_Line)) &&
BRep_Tool::Degenerated(E)) {
// The edge 2D Geometry degenerates in 3D
// The edge 2D geometry is not mapped onto any Step entity
// (ProStep agreement)
continue;
}
else { // Copy the Curve2d which might be changed
//C2d = CA.CurveOnSurface().GetCurve().Curve();
//C2d = Handle(Geom2d_Curve)::DownCast(C2d->Copy());
C2d = Handle(Geom2d_Curve)::DownCast(C2d->Copy()); }
...

It appears that edge E is not degenerated, but C2d is a null curve. I don't believe that this is supposed to happen. Very likely the bug is not actually at this line of code, but is in the STEP model reading code somewhere. Has anyone else seen this or does anyone know how to fix it?

-- Thanks, Shaun