Can't Make Mesh Display Using Quill18 Tilemap Tutorial (Boo conversion)

I'm working through the tilemap tutorials by Quill18, video 2, and since I'm coming mainly from a Python background I'd like to work in Boo. I think I've translated the code (and have tried just using his C# code), but I'm not seeing the rendered square that should be appearing. The code below is attached to a GameObject with a camera pointed at it. Judging from the tutorial video, it should be displaying a pink square (until a texture is added) but I see nothing at all.

Also, Unity is showing an error: "NullReferenceException: Object reference not set to an instance of an object. TileMap.BuildMesh() (at Assets/Tilemap.boo:21)". That line appears to be an assignment: "vertices[0] = Vector3(0,0,0)".

Does anyone know what's gone wrong, here?

 import UnityEngine
 
 ## Force these components to be created on the target object.
 [RequireComponent(MeshFilter)]
 [RequireComponent(MeshCollider)]
 [RequireComponent(MeshRenderer)]
 
 class TileMap (MonoBehaviour): 
 
     def Start():
         BuildMesh()
 
     def BuildMesh():
         vertices as (Vector3)
         triangles as (int)
         normals as (Vector3)
         uv as (Vector2)
         
         ## Generate mesh.
         
         vertices[0] = Vector3(0,0,0)
         vertices[1] = Vector3(1,0,0)
         vertices[2] = Vector3(0,0,-1)
         vertices[3] = Vector3(1,0,-1)
         
         triangles = (0,3,2,0,1,3)
 
         for n in range(4):
             normals[n] = Vector3.up
 
         uv[0] = Vector2(0,0)
         uv[1] = Vector2(1,0)
         uv[2] = Vector2(0,1)
         uv[3] = Vector2(1,1)        
 
         ## Original code was just the next line, but
         ## the following two are suggested by the official reference.        
         mesh = Mesh()
 #        mesh as Mesh = Mesh()
 #        GetComponent[of MeshFilter]().mesh = mesh
 
         ## Assign mesh data to mesh.                        
         mesh.vertices = vertices
         mesh.triangles = triangles
         mesh.normals = normals
         mesh.uv = uv
 
         mesh_filter = GetComponent(MeshFilter)
 
         mesh_filter.mesh = mesh
 
     def Update ():
         pass