I'm currently using the PPVcube function to make FITS files of a gizmo simulation. I would like the field used to make the cubes to be mass rather than density. The cube is created however, when I try to write the cube, I get the error copied below. I included the code directly before it.  

Is it not possible to write FITS cubes with other fields?

Cheers,

Sam

-------
KeyError                                  Traceback (most recent cal
l last)
<ipython-input-9-4e7ff69a0d0f> in <module>()
----> 1 cubeL.write_fits("mass.fits",length_unit="kpc", clobber='Tru
e')
/anaconda2/lib/python2.7/site-packages/yt/utilities/parallel_tools/p
arallel_analysis_interface.pyc in root_only(*args, **kwargs)
317
318
--> 319
320 321
def root_only(*args, **kwargs):
    if not parallel_capable:
        return func(*args, **kwargs)
    comm = _get_comm(args)
    rv = None
/anaconda2/lib/python2.7/site-packages/yt/analysis_modules/ppv_cube/
ppv_cube.pyc in write_fits(self, filename, overwrite, length_unit, s
ky_scale, sky_center, **kwargs)
313
314
--> 315
317 fib.update_all_headers("btype", self.field)
/anaconda2/lib/python2.7/site-packages/yt/visualization/fits_image.p
yc in __init__(self, data, fields, units, width, wcs)
                w.wcs.ctype = ["LINEAR","LINEAR",vtype]
                fib = FITSImageData(self.data.transpose(), fields=se
                fib.update_all_headers("bunit", re.sub('()', '', str
(self.proj_units)))
lf.field, wcs=w)316
158
159
--> 160
161
eld].units)162
KeyError: 'cell_mass'