nanmean and nanvar raise RuntimeWarning exceptions that are not called by their non-nan counterpart functions.

Reproducing code example:

import numpy as np

test = np.zeros((2, 3, 4))
test[:,:,1:2] = np.nan

np.mean(test, axis=0)    # OK
np.nanmean(test, axis=0) # RuntimeWarning: Mean of empty slice

np.var(test, axis=0)     # OK
np.nanvar(test, axis=0)  # RuntimeWarning: Degrees of freedom <= 0 for slice.

Error message:

RuntimeWarning: Mean of empty slice
RuntimeWarning: Degrees of freedom <= 0 for slice.

Numpy/Python version information:

1.17.2 3.7.4 (default, Aug 13 2019, 15:17:50) [Clang 4.0.1 (tags/RELEASE_401/final)]