Mesh topological identical vertices

When does mesh.Vertices.GetTopologicalIndenticalVertices(vertexID); returns null?

File and simple mesh attached.

http://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Geometry_Collections_MeshVertexList_GetTopologicalIndenticalVertices.htm

TopologicalIndeticalVertices.gh (7.7 KB)

Got it no need for the answer… I had in mind x.TopologyVertices.TopologyVertexIndex(y)