Inconsistent FloatingPointError when comparing NaNs

[al626]

When doing np.seterr(invalid='raise'), FloatingPointErrors are raised when comparing NaNs of array length greater than 1, but not when comparing NaNs of array length equal to 1. It's easiest to see what's happening by looking at the code example:

import numpy as np
np.seterr(invalid='raise')
# {'over': 'warn', 'under': 'ignore', 'invalid': 'warn', 'divide': 'warn'}
np.array([np.nan]) < 0
# array([False], dtype=bool)
np.array([np.nan, np.nan]) < 0
# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# FloatingPointError: invalid value encountered in less

System info:

$ uname -a
Linux a 4.4.0-78-generic #99~14.04.2-Ubuntu SMP Thu Apr 27 18:49:46 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
$ python -c 'import numpy; print numpy.__version__'
1.13.0
$ python --version
Python 2.7.6

[al626]

dupe of #7352