Some calls to acs5.state_county_tract() return the value -666666666. This seems like a semantically significant value, but I can't find documentation on what it means.

For a reproducible example, this call returns the value in question:

>>> c = Census(os.environ['CENSUS_API_KEY'], year=2016)
>>> c.acs5.state_county_tract('B19081_001E', 42, 101, '989100')

[{'B19081_001E': -666666666.0, 'state': '42', 'county': '101', 'tract': '989100'}]

Any thoughts on what this response means, and how I should handle it? So far I've just been converting them to nulls.