Type Checker inspection for function `str.casefold`

I'm scratching my head a bit on this. PyCharm is trying to tell me that there is no casefold function in my Python 3.4 source code.

In a python scratch file I get a type checker warning on the 'HEllo' string: 

def do_casefold(s):
    return s.casefold()
t = do_casefold('HEllo')
print(t)

If I put in a type annotation, then the warning moves to the casefold function call. 

def do_casefold(s: str):
    return s.casefold()
t = do_casefold('HELLo')
print(t)

I've tried a few other str built ins, but this one is the only one I've found that PyCharm doesn't seem to know about this. 

Is there a cache that I need to rebuild, or has nobody told PyCharm about this (relatively new) function?

Thanks, 

Seth