arbitrary grid field error

In [37]: data=ug["Density"]
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-37-25a1a12e7337> in <module>()
----> 1 data=ug["Density"]

/opt/apps/yt/yt-x86_64/src/yt-hg/yt/data_objects/data_containers.py in __getitem__(self, key)
    251                 return self.field_data[f]
    252             else:
--> 253                 self.get_data(f)
    254         # fi.units is the unit expression string. We depend on the registry
    255         # hanging off the dataset to define this unit object.

/opt/apps/yt/yt-x86_64/src/yt-hg/yt/data_objects/construction_data_containers.py in get_data(self, fields)
    600         fill, gen, part, alias = self._split_fields(fields_to_get)
    601         if len(part) > 0: self._fill_particles(part)
--> 602         if len(fill) > 0: self._fill_fields(fill)
    603         for a, f in sorted(alias.items()):
    604             self[a] = f(self)

/opt/apps/yt/yt-x86_64/src/yt-hg/yt/data_objects/construction_data_containers.py in _fill_fields(self, fields)
    785
    786     def _fill_fields(self, fields):
--> 787         raise NotImplementedError
    788
    789 class LevelState(object):

NotImplementedError: