DOLFIN can't find point inside mesh

Hi,

I'm trying to solve Poisson's equation with a variable conductivity, sigma, and I'm overriding the "eval" function of Expression to
do so. For some reason the interpolated "sigma" can't return values at seemingly random points inside the domain.
A minimal working script is given bellow as well as the error messages I get with and without extrapolation.

Is there any way around this problem?

Error with "parameters['allow_extrapolation'] = False":

Evaluating at x = <Point x = 0.233 y = 0.638 z = 0.039>
Traceback (most recent call last):
  File "minimal_debug.py", line 26, in <module>
    print sigma(0.233, 0.638, 0.039)
  File "/usr/lib/python2.7/dist-packages/dolfin/functions/function.py", line 382, in __call__
    self.eval(values, x)
RuntimeError:

*** -------------------------------------------------------------------------
*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at
***
*** https://answers.launchpad.net/dolfin
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error: Unable to evaluate function at point.
*** Reason: The point is not inside the domain. Consider setting "allow_extrapolation" to allow extrapolation.
*** Where: This error was encountered inside Function.cpp.
*** Process: 0
*** -------------------------------------------------------------------------

Error with "parameters['allow_extrapolation'] = True":

Traceback (most recent call last):
  File "minimal_debug.py", line 27, in <module>
    print sigma(0.233, 0.638, 0.039)
  File "/usr/lib/python2.7/dist-packages/dolfin/functions/function.py", line 382, in __call__
    self.eval(values, x)
RuntimeError:

*** -------------------------------------------------------------------------
*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at
***
*** https://answers.launchpad.net/dolfin
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error: Unable to perform call to DOLFIN function.
*** Reason: The function compute has not been implemented (in /build/buildd/dolfin-1.2.0+dfsg/dolfin/intersection/IntersectionOperatorImplementation.h line 302).
*** Where: This error was encountered inside log.h.
*** Process: 0
*** -------------------------------------------------------------------------

from dolfin import *

class DebugConductivity(Expression):
    """ This inherits from ConductivityClass and contains the specific
    conductivity of the control set-up"""

    def eval(self, value, x):
        """ This function is written like this so I can use return_conductivity() outside the class for
        debugging reasons """
        value[0] = 1

parameters['allow_extrapolation'] = True
N = 25
mesh = BoxMesh(0, 1, 0, 1, 0, 1, N, N, N)

V = FunctionSpace(mesh, "CG", 2)
Vs = FunctionSpace(mesh, "DG", 0) # For conductivity tensor
sigma = Function(Vs)

Cond = DebugConductivity()
sigma.interpolate(Cond)

Cheers,
Torbjørn