Unable to Convert geometry to polyhedron

import dolfin
import mshr

dolfin.set_log_level(dolfin.TRACE)

c = mshr.Sphere(dolfin.Point(0, 0, 0), 1)
c = c - mshr.Sphere(dolfin.Point(0, 0, 0), 0.8)
cm1 = mshr.generate_mesh(c, 5)

throws the following exception

Convert to nef polyhedron
Traceback (most recent call last):
  File "scripts/meshdemo.py", line 20, in <module>
    cm1 = mshr.generate_mesh(c, resolution)
  File "/home/thomas/miniconda/envs/py27_fenics15dev_petsc35/lib/python2.7/site-packages/mshr.py", line 549, in generate_mesh
    _generate(m, geometry, resolution, backend)
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
***
***     fenics@fenicsproject.org
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error:   Unable to Convert geometry to polyhedron.
*** Reason:  Geometry contains no facet.
*** Where:   This error was encountered inside CSGCGALDomain3D.cpp.
*** Process: unknown
*** 
*** DOLFIN version: 1.4.0+
*** Git changeset:  a2e03548dc52b5f0c15394efaeb00ad01ec346df
*** -------------------------------------------------------------------------

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