Create U/V Curve, but errors

hohox

I use Body.CreateParametricCurveU(Face face, double vParam) to create a U Curve on a spline face. and the vParam is in the range of face.GetVRange(ParameterRange), just between ParameterRange.start and ParameterRange.end.

I can get the VRange Parameter. but when I Body.CreateParametricCurveU(Face face, double vParam), it returns null, no curve is created. and RobotStudio gives the error message: Geometry System (ACIS) Error code: 87003 "attempt to fit curve to zero knot span".

 

Where is wrong? And how can I Create a U or V curve on a selected face?

 

Thanks very much!

hohox

BTW, I do these in an Addin program, and use RS 5.11