Poor error message on Dataset.sum(axis=...)

ll_means.nc.gz

MCVE Code Sample

I'm attaching the relevant .nc file.

foo = xr.open_dataset('ll_means.nc')
print(foo)
foo.sum(axis=1) # or foo.sum(axis=0)

Instead of getting a sum (either across the data variables or across the dimension shared between the variables), I get the error in the title: ValueError: cannot supply both 'axis' and 'dim' arguments even though I did not supply a dim argument. Furthermore, I get this error even when passing dim=None.

It's possible that I am doing something wrong, and it is just the error message that is bad, and not the behavior. I don't know enough to tell.

Expected Output

TBQH, I didn't know whether this would sum across the (single) dimension of this dataset, or if it would sum across the data variables along that dimension. I was experimenting to try to figure this out.

What I am trying to do is sum across the data variables, "perpendicular" to a dimension, instead of along it.

Problem Description

I assumed that summing across one of the axes would sum across the data variables, but perhaps that was a bad assumption. At any rate, the current behavior is undesirable.

Output of xr.show_versions()

Start with the MCVE entry point, Dataset.sum(axis=...), using the attached ll_means.nc file and the reported axis=0, axis=1, and dim=None calls. Determine whether the issue is the operation or only the error message; done means the requested behavior is defined and the resulting error or sum is consistent with that definition.