Adding mesh to document returns guid 00000000-0000-0000-0000-000000000000 ???

Hi,

I'm having some problems with the following lines of code.

msh = rs.JoinMeshes ((msh,newMsh), True)
rs.UnifyMeshNormals(msh)

mesh = rs.coercemesh(msh)
Rhino.Geometry.Collections.MeshVertexList.CombineIdentical(mesh.Vertices, True, True)
mesh = sc.doc.Objects.AddMesh(mesh)

I run this because rs.JoinMeshes doesn't seem to weld the vertices, and there doesn't appear to be any weld method in rhino python. This works most of the time, but sometimes it returns a mesh with guid 00000000-0000-0000-0000-000000000000, which I'm guessing is different than returning False, but it is not a valid mesh? Can someone help me understand what is happening?

Thanks! files attached.