'NoneType' object is not iterable exception for yyyy/m/d date format

[alvaromorales]

Dates of format yyyy/m/d cause a TypeError. This input format should be handled, or a more specific exception should be raised.

>>> from overlay_parse import dates
>>> dates.just_dates('1821/5/5')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "build/bdist.linux-x86_64/egg/overlay_parse/dates.py", line 282, in just_dates
  File "build/bdist.linux-x86_64/egg/overlay_parse/dates.py", line 271, in just_props
  File "build/bdist.linux-x86_64/egg/overlay_parse/overlays.py", line 189, in overlay
  File "build/bdist.linux-x86_64/egg/overlay_parse/matchers.py", line 182, in offset_overlays
  File "build/bdist.linux-x86_64/egg/overlay_parse/matchers.py", line 151, in _merge_ovls
  File "build/bdist.linux-x86_64/egg/overlay_parse/matchers.py", line 23, in value
  File "build/bdist.linux-x86_64/egg/overlay_parse/dates.py", line 48, in date_conditional
TypeError: 'NoneType' object is not iterable