why doesn't the websocket example work? I get this error: on_message() takes 2 positional arguments but 3 were given

import poloniex
import logging
from time import sleep

logging.basicConfig()
poloniex.logger.setLevel(logging.DEBUG)

def on_volume(data):
print(data)

sock = poloniex.PoloniexSocketed()

sock.startws(subscribe={'24hvolume': on_volume})

sleep(5)

sock.subscribe(chan='ticker', callback=print)
sleep(1)

sock.unsubscribe(1002)
sleep(4)

sock.stopws()