psubscribe scheduled to be closed ASAP for overcoming of output buffer limits

this error message has been logged. It has only happened once in about 2 months. We have a sub pub client listening for Redis events. What kind of problem could be causing it? What actions can we take to mitigate it?

Here is the detailed error:

Client id=599475 addr=10.250.0.50:44012 fd=1370 name= age=18039 idle=1 flags=N db=0 sub=0 psub=1 multi=-1 qbuf=0 qbuf-free=0 obl=0 oll=555 omem=9085669 events=rw cmd=psubscribe scheduled to be closed ASAP for overcoming of output buffer limits.

Redis is configured to post events to the pub sub using the notify-keyspace-events setting to "AKE". The code used (python) is like this:

#... 
redis: StrictRedis = StrictRedis(config.REDIS_HOST, config.REDIS_PORT)
pubsub = redis.pubsub()  
pubsub.psubscribe("keyevent@0:*")
PAUSE = True
while PAUSE:
    message = pubsub.get_message()
    if message:
        #.. do something 
        time.sleep(0.001)