Are double underscore string filters supposed to work for custom fields?

[sdktr]

Custom Field lookup expression filtering (double underscore) doesn't work

1. Create a custom field named 'test' of the type text, add it to Device object
2. on device1, fill the field with value 'foo'
3. Filter Device list using one of the double underscore syntax options::
   http://netbox/dcim/devices?cf_test__empty=False

Expected:
device1 is filtered from the results, depending on the lookup expression

Result:
device1 is included in the results

All the __ lookup expressions fail to filter. The regular '=' syntax works correctly on custom text fields.

Are custom fields excluded from filtering this way 'by design' or is this a bug?