`'NoneType' object has no attribute 'is_ambiguous'` when using `today().add(years=-50)`

[dekoza]

Lib versions:
pendulum==2.04

How to reproduce:
pendulum.today().add(years=-50)

Risen exception:

/usr/local/lib/python3.6/site-packages/pendulum/tz/timezone.py in _normalize(self, dt, dst_rule)
    109                 transition = transition.previous
    110 
--> 111         if transition.is_ambiguous(sec):
    112             # Ambiguous time
    113             if dst_rule == TRANSITION_ERROR:

AttributeError: 'NoneType' object has no attribute 'is_ambiguous'

[pR0Ps]

I'm pretty sure this can be closed - it seems fixed by #321 and #350