Incorrect Pluralization of '-ies' words, collective nouns

[tbpassin]

I discovered that the Inflector does not return the plural form of certain words when given a plural form. For some, it actually returns the singular form. The ones I know about so far are:

Word            Returns            Desired
-----------------------   
berries         berry                  berries
secretaries   secretary           secretaries
oxen            oxens                oxen 
mice            mices                mice

Tested as follows on Python 2.7, Windows 10:

from Pluralizer.inflector import Inflector
>>> infl = Inflector()
>>> for wd in 'mice oxen berries secretaries'.split(): print wd, infl.pluralize(wd)
...
mice mices
oxen oxens
berries berry
secretaries secretary

Expected behavior: pluralize() should return the input word when it is plural already.

So far, I have not found any corresponding errors with the singularize() method.