Missing elevation in isd-history.txt

[TPetroski]

ftp://ftp.ncdc.noaa.gov/pub/data/noaa/isd-history.txt
691810 99999 TUZLA BK KQLH +44.533 +018.717 19960221 20030610

This record in isd-history.txt has not elevation (currently 98 stations) which results in this error when using station.py:

Error:
nearest_isd_stations = Station.nearest(self.lat,self.lon,radius=self.radius,limit=self.limit)
File "/usr/local/lib/python2.7/site-packages/skywisehistoricalobs/station.py", line 49, in nearest
return Station.find(latitude=latitude, longitude=longitude, **kwargs)
File "/usr/local/lib/python2.7/site-packages/skywiserestclient/skywise.py", line 190, in find
return cls._unpack_response(r)
File "/usr/local/lib/python2.7/site-packages/skywiserestclient/skywise.py", line 459, in _unpack_response
resource._load_json(j)
File "/usr/local/lib/python2.7/site-packages/skywiserestclient/skywise.py", line 470, in _load_json
raise SkyWiseJSONDeserializationException(err)
SkyWiseJSONDeserializationException: The response for your requested resource returned an invalid format or data value. The response is as follows: {"distance": 2.322355733, "latest_recorded_at": "2003-06-10T00:00:00Z", "description": "TUZLA BK KQLH", "earliest_recorded_at": "1996-02-21T00:00:00Z", "location": {"latitude": 44.533, "elevation": null, "longitude": 18.717}, "id": "691810-99999"}