water drain vb script error

i am new to the vb.net in grasshopper ,since i start to learn it.and i am trying study it from this example:http://issuu.com/pabloherrera/docs/vb-workshop-harvard-gsd-woojsung... .but i met an error and have no idea what happen.coule somebody help me!!!

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firstscript.gh, 5 KB

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Permalink Reply by David Rutten on January 21, 2015 at 5:28am

You need to do some null checking to make sure that your variables actually exist.

I'd also recommend getting the casing of your types and method names right. VB is not case-sensitive, but other languages are so it makes translating code between languages a lot more hassle:

If (basesrf Is Nothing) Then Return
If (Not input_pt.IsValid) Then Return

Dim u, v As Double
basesrf.ClosestPoint(input_pt, u, v)

Dim normal As Vector3d
normal = basesrf.NormalAt(u, v)

Dim drain As vector3d = Vector3d.CrossProduct(normal, Vector3d.ZAxis)
If (Not drain.IsValid) Then Return
If (drain.IsZero) Then Return

drain.Unitize()
drain.Transform(Transform.Rotation(Math.PI * 0.5, normal, input_pt))

Dim moved_pt As Point3d = input_pt + distance * drain
basesrf.ClosestPoint(moved_pt, u, v)

Dim output_pt As Point3d = basesrf.PointAt(u, v)
A = output_pt

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