AutoCAD .NET :: Getting Y Intersection Of A Line?

I'm trying to get the "Y" intersection of a line I construct and a given polyline using the following code.The debugger stops at this line that calls the function

intersectStartY = GetYIntersection(elevationLine.StartPoint)

The error message is "MissingMethodException was unhandled by user code"

Method not found: 'Void Autodesk.AutoCAD.DatabaseServices.Entity.IntersectWith(Autodesk.AutoCAD.DatabaseServices.Entity, Autodesk.AutoCAD.DatabaseServices.Intersect, Autodesk.AutoCAD.Geometry.Point3dCollection, Int32, Int32)'.
{code}
Public Function GetYIntersection(ByVal elevationLinePoint As Point3d) As Double
Dim intersectY As Double = Nothing
Dim intersectPoints As Point3dCollection = Nothing

[code].....