circle to polyline loft problem

[jonk]

I am having a problem with the code below. I have a list of circles
and a list of polylines and I want to loft the first polyline to the
first circle and the second polyline to the second circle and so
on...seems simple enough. The problem is that I get an error saying
that DuplicateCurve() is not a method of OnCircle ( which of course is
true). Any suggestions? I have looked all over this site but haven't
found a similar problem. thx.
jon

Sub RunScript(ByVal circles As List(Of OnCircle), ByVal cells As List
(Of OnCurve))
    'Declare loop variable
    Dim i As Integer
    Dim count As Integer
    count = circles.Count() - 1
    Dim lfts As New List(Of OnNurbsSurface)

    For i = 0 To count
      Dim loftCurves As New List(Of MRhinoLoftCurve)

      Dim topCurve As New MRhinoLoftCurve()
      topCurve.m_curve = circles(i).DuplicateCurve()
      loftCurves.Add(topCurve)

      Dim bottomCurve As New MRhinoLoftCurve()
      bottomCurve.m_curve = cells(i).DuplicateCurve()
      loftCurves.Add(bottomCurve)

      Dim loftArgs As New MArgsRhinoLoft()
      loftArgs.m_loftcurves = loftCurves.ToArray()
      loftArgs.m_bClosed = False
      Dim surfaceList(0) As OnNurbsSurface

      Call  RhUtil.RhinoSdkLoftSurface(loftArgs, surfaceList)

      lfts.Add(surfaceList(0))
    Next

    loftedSurfaces = lfts

  End Sub