ON_Curve::Ev1Der() get tangent error?

Rhino Developer
1

Hi all,
One normal line in Debug.3dm
Debug.rar (3.3 MB)

I use ON_Curve::Ev1Der() to get tangent vector.
The vertex is correct.

Noname2.png742×135 7.16 KB

Why the tangent vector = (0,0,0)?

Noname3.png770×139 6.29 KB

Is the incorrect curve data?
How do I fix the curve?

Thanks

2

There is no tangent defined because there are two control points stacked on top of each other.

3

Thank for your help.