ON_Curve::Ev1Der() get tangent error?

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.

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

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


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

Thank for your help.