TypeError when initializing Nco object using python/pynco

Question

I am new to using using NCO tools and their Python bindings for working with netCDF files. I have successfully installed the NCO tools and the pynco package (using pip) following the instructions here. But I am getting an error when I try to initialize an object.

Here are the commands I'm using:

from nco import Nco
nco = Nco()

And the error is:

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "C:\Python27\ArcGIS10.5\lib\site-packages\nco\nco.py", line 54, in __init__
    self.NCOpath = os.path.split(which('ncks'))[0]
  File "C:\Python27\ArcGIS10.5\lib\ntpath.py", line 180, in split
    d, p = splitdrive(p)
  File "C:\Python27\ArcGIS10.5\lib\ntpath.py", line 115, in splitdrive
    if len(p) > 1:
TypeError: object of type 'NoneType' has no len()

This suggests to me that the install may not have worked properly, though I can run NCO commands from the command line.

Any idea what the problem may be?