Traceback "gas", "HII_Density"

>>> objfield1=obj["gas", "HII_Density"]
Parsing Hierarchy : 100%|##############################################| 5568/5568 [00:00<00:00, 15594.54it/s]
yt : [INFO     ] 2017-07-21 12:07:48,268 Gathering a field list (this may take a moment.)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/data_containers.py", line 274, in __getitem__
    f = self._determine_fields([key])[0]
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/data_containers.py", line 1121, in _determine_fields
    finfo = self.ds._get_field_info(ftype, fname)
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/static_output.py", line 751, in _get_field_info
    raise YTFieldNotFound((ftype, fname), self)
yt.utilities.exceptions.YTFieldNotFound: Could not find field '('gas', 'HII_Density')' in RedshiftOutput0001.

>>> objfield2=obj["gas", "HI_Density"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/data_containers.py", line 274, in __getitem__
    f = self._determine_fields([key])[0]
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/data_containers.py", line 1121, in _determine_fields
    finfo = self.ds._get_field_info(ftype, fname)
  File "/opt/apps/yt/3.3.5a/yt-conda/src/yt-git/yt/data_objects/static_output.py", line 751, in _get_field_info
    raise YTFieldNotFound((ftype, fname), self)
yt.utilities.exceptions.YTFieldNotFound: Could not find field '('gas', 'HI_Density')' in RedshiftOutput0001.