When call IG.sweep two lines, the generated surface is invalid

[chatyan]

Dear @sghr San,
Your library is very good, it's like magic for creating 3D models using Java.

Currently, I am facing a problem (maybe it's my misunderstanding of the usage).
I tried to sweep two curves. Everything is OK.
But when I sweep two straight lines, which are generated by ICurveGeo sharing the same z-coordinate,
I got some error messages like the surface is invalid. I provide the following code, hope you can reproduce it.

`import processing.opengl.;
import igeo.;

size(480, 360, IG.GL);
IVec[] cptsU = new IVec[5];
cptsU[0] = new IVec(0, 0, 50);
cptsU[1] = new IVec(60, 0, 50);
cptsU[2] = new IVec(120, 0, 50);
cptsU[3] = new IVec(180, 0, 50);
cptsU[4] = new IVec(240, 0, 50);
ICurveGeo m_curveU = new ICurveGeo(cptsU, 3, false);

IVec[] cptsV = new IVec[5];
cptsV[0] = new IVec(100, 0, 10);
cptsV[1] = new IVec(100, 50, 10);
cptsV[2] = new IVec(100, 100, 10);
cptsV[3] = new IVec(100, 150, 10);
cptsV[4] = new IVec(100, 200, 10);
ICurveGeo m_curveV = new ICurveGeo(cptsV, 3, false);

IG.sweep(m_curveU, m_curveV);
`
In addition, if I misunderstood IG.sweep, can you correct me, or guide me another way to achieve a surface by sweeping two straight lines?
Sincerely！