GetDistAtPoint and einvalidinput error

Hi there;

I'm trying to find distance of a specified point on a 3d polyline with "getdistatpoint" function. sometimes it works properly but sometimes it gives einvalidinput error.

I searched web and found this function:

   Public Shared Function GetDistanceToPoint(curve As Curve, pt As Point3d) As Double
       Dim ptOnCurve As Point3d = curve.GetClosestPointTo(pt, False)
       Dim ptparam As Double = curve.GetParameterAtPoint(ptOnCurve)
       Dim a As Double = curve.GetDistanceAtParameter(ptparam)
       Dim b As Double = curve.GetDistanceAtParameter(curve.StartParam)
       Return a - b
   End Function

This works better than "getdistatpoint" function but it gives "einvalidinput" error too.

I really need help, thanks in advance.