AttributeError: 'FeedExporter' object has no attribute 'slot'

I have a spider that suddenly throws this exception on about a half of by requests:

[scrapy.utils.signal] Error caught on signal handler: <bound method FeedExporter.item_scraped of <scrapy.extensions.feedexport.FeedExporter object at 0x7fde660b5978>> Less

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/twisted/internet/defer.py", line 151, in maybeDeferred
    result = f(*args, **kw)
  File "/usr/local/lib/python3.7/site-packages/pydispatch/robustapply.py", line 55, in robustApply
    return receiver(*arguments, **named)
  File "/usr/local/lib/python3.7/site-packages/scrapy/extensions/feedexport.py", line 264, in item_scraped
    slot = self.slot
AttributeError: 'FeedExporter' object has no attribute 'slot'

 I'm exporting my results to AWS S3 like so in settings.py:

S3_BUCKET = "s3://my-bucket"
S3_REGION = "eu-west-1"
FEED_FORMAT = "csv"
FEED_EXPORT_ENCODING = "utf-8"
FEED_URI = S3_BUCKET + "/%(name)s/%(time)s.csv"

The spider runs as expected locally and did so previously on ScrapingHub as well.

Any clues on how to debug?