Sherpa.astro.ui module fails to load

gistfile1.pytb

In [33]: import sherpa.astro.ui as ui
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)

/disk/caleuche_1/trive/Arbejde/LARS-COS/<ipython console> in <module>()

/home/trive/Softwarebuild/python_packages/lib/python2.6/site-packages/sherpa/astro/ui/__init__.py in <module>()
     19 
     20 import sherpa.all
---> 21 import sherpa.astro.all
     22 import sherpa.astro.ui.utils
     23 from sherpa.utils import calc_mlr, calc_ftest, rebin, histogram1d, \

/home/trive/Softwarebuild/python_packages/lib/python2.6/site-packages/sherpa/astro/all.py in <module>()
     23 from sherpa.astro.background import *
     24 from sherpa.astro.data import *
---> 25 from sherpa.astro.instrument import *
     26 from sherpa.astro.flux import *
     27 

/home/trive/Softwarebuild/python_packages/lib/python2.6/site-packages/sherpa/astro/instrument.py in <module>()
     22 from sherpa.models.model import ArithmeticFunctionModel, NestedModel, \
     23     ArithmeticModel, CompositeModel, Model
---> 24 from sherpa.astro.io.wcs import WCS
     25 from sherpa.instrument import PSFModel as _PSFModel
     26 from sherpa.utils import NoNewAttributesAfterInit

/home/trive/Softwarebuild/python_packages/lib/python2.6/site-packages/sherpa/astro/io/wcs.py in <module>()
     20 import numpy
     21 from sherpa.utils import NoNewAttributesAfterInit
---> 22 from sherpa.astro.utils._wcs import pix2world, world2pix
     23 
     24 class WCS(NoNewAttributesAfterInit):

ImportError: No module named _wcs

In [34]: