When i run the code
A = np.full(7, np.nan)
there is no issue.

When i run
A = np.full(8, np.nan)
and for any array larger than that, i get the warnings:

/mnt/home/[...]/anaconda3/lib/python3.6/site-packages/numpy/core/_methods.py:29: RuntimeWarning: invalid value encountered in reduce
return umr_minimum(a, axis, None, out, keepdims)
/mnt/home/[...]/anaconda3/lib/python3.6/site-packages/numpy/core/_methods.py:26: RuntimeWarning: invalid value encountered in reduce
return umr_maximum(a, axis, None, out, keepdims)

i use spyder 3.2.6, python 3.6.4, numpy 1.14.0
I have this warning on one of my computers (running CentOS Linux 7), but not on the other (running Ubuntu 16.04)
Cannot figure out where the different comes from. Any ideas?