Boundaries are marked only partially

Hi everyone,
I'd like to use a MeshFunction to hold markers for the vertical boundaries of a domain. Inspecting the marked MeshFunction (with plot() and outputting it in vtk format), it seems that the vertical boundary gets marked only partially. It happens regardless of the size, number of elements, geometry (Rectangle, UnitSquare), side...
I've reproduced the issue on two machines with the code that follows.
Any suggestion about what is going wrong?

Cheers,

Andrés León Baldelli

ps: I'm using the stable version of FEniCS built with dorsal on a Darwin10.8 machine.

_____Begin Python Snippet______

from dolfin import *
import numpy as np
l = 1. # width of the cell
nx = 12 # n of elts in x[0] direction
mesh = UnitSquare(nx, nx)

class Left(SubDomain):
 def inside(self, x, on_boundary):
  return abs(x[0] - 1.) < DOLFIN_EPS and on_boundary

boundaries = MeshFunction("uint", mesh, mesh.topology().dim()-1)
boundaries.set_all(0)

G_Left = Left()

G_Left.mark(boundaries, 1)
plot(boundaries, interactive=True)

_____End Python Snippet______