'`~astropy.units.Unit` : The unit of the uncertainty, if any.
Even though it is not enforced the unit should be convertible to the
``parent_nddata`` unit. Otherwise uncertainty propagation might give
wrong results.
If the unit is not set the unit of the parent will be returned.'

@property
def unit(self):

    if (self._unit is None):
        if ((self._parent_nddata is None) or (self.parent_nddata.unit is None)):
            return None
        else:
            return self.parent_nddata.unit
    return self._unit
