Plyer TTS not working on Windows

I cannot get plyer.tts to work on Windows.

Installed:

plyer 1.3.0

python 3.6.5

Example:

>>> from plyer import tts
>>> tts.speak("Hello world!")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\user\AppData\Local\Programs\Python\Python36\lib\site-packages\plyer\facades\tts.py", line 30, in speak
    self._speak(message=message)
  File "C:\Users\user\AppData\Local\Programs\Python\Python36\lib\site-packages\plyer\facades\tts.py", line 35, in _speak
    raise NotImplementedError()
NotImplementedError

The documentation states that tts works on Windows and I remember getting it to work in the past on a different PC.

What could be the problem?