How to handle "Redis.exceptions.ConnectionError: Connection has data"

I receive following output:

Traceback (most recent call last):
  File "/home/ec2-user/env/lib64/python3.7/site-packages/redis/connection.py", line 1192, in get_connection
    raise ConnectionError('Connection has data')
redis.exceptions.ConnectionError: Connection has data

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/ec2-user/env/lib64/python3.7/site-packages/eventlet/hubs/hub.py", line 457, in fire_timers
    timer()
  File "/home/ec2-user/env/lib64/python3.7/site-packages/eventlet/hubs/timer.py", line 58, in __call__
    cb(*args, **kw)
  File "/home/ec2-user/env/lib64/python3.7/site-packages/eventlet/greenthread.py", line 214, in main
    result = function(*args, **kwargs)
  File "crawler.py", line 53, in fetch_listing
    url = dequeue_url()
  File "/home/ec2-user/WebCrawler/helpers.py", line 109, in dequeue_url
    return redis.spop("listing_url_queue")
  File "/home/ec2-user/env/lib64/python3.7/site-packages/redis/client.py", line 2255, in spop
    return self.execute_command('SPOP', name, *args)
  File "/home/ec2-user/env/lib64/python3.7/site-packages/redis/client.py", line 875, in execute_command
    conn = self.connection or pool.get_connection(command_name, **options)
  File "/home/ec2-user/env/lib64/python3.7/site-packages/redis/connection.py", line 1197, in get_connection
    raise ConnectionError('Connection not ready')
redis.exceptions.ConnectionError: Connection not ready

I couldn’t find any issue related to this particular error. I emptied/flushed all redis databases, so there should be no data there. I assume it has something to do with eventlet and patching. But even when I put following code right at the beginning of the file, the error appears.

import eventlet
eventlet.monkey_path()

What does this error mean?