FieldList.select_by_ncvars throws an error

I get the following error (using cf-python 3.4.0) when trying to use FieldList.select_by_ncvars to select a field by netCDF variable name:

In [1]: import cf                                                                                                                                                                                 
 
In [2]: fl = cf.read('ESACCI-OC-L3S-IOP-MERGED-5D_DAILY_4km_GEO_PML_OCx_QAA-20010101-fv4.2.nc')                                                                                                   
 
In [3]: fl.select_by_ncvar('atot_412')                                                                                                                                                            
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-3-0069b9e04a8b> in <module>
----> 1 fl.select_by_ncvar('atot_412')
 
~/miniconda3/envs/cci/lib/python3.7/site-packages/cf/fieldlist.py in select_by_ncvar(self, *rank)
   1044 
   1045         '''
-> 1046         return type(self)(f for f in self if f.match_by_ncvar(*ncvars))
   1047 
   1048     def select_by_property(self, *mode, **properties):
 
~/miniconda3/envs/cci/lib/python3.7/site-packages/cf/fieldlist.py in __init__(self, fields)
     54                 self.append(fields)
     55             else:
---> 56                 self.extend(fields)
     57 
     58     def __call__(self, *identities):
 
~/miniconda3/envs/cci/lib/python3.7/site-packages/cf/fieldlist.py in <genexpr>(.0)
   1044 
   1045         '''
-> 1046         return type(self)(f for f in self if f.match_by_ncvar(*ncvars))
   1047 
   1048     def select_by_property(self, *mode, **properties):
 
NameError: name 'ncvars' is not defined

I tried this too:

In [6]: fl.select_by_ncvar(ncvars='atot_412')                                                                                                                                                     
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-6-37ac3193cda2> in <module>
----> 1 fl.select_by_ncvar(ncvars='atot_412')
 
TypeError: select_by_ncvar() got an unexpected keyword argument 'ncvars'

Please would you be able to help?