ic-py: TypeError: object of type 'NoneType' has no len()

When I run this code

from ic.client import Client
from ic.identity import Identity
from ic.agent import Agent
from ic.candid import encode, Types
import time
import schedule
import sys

# params is an array, return value is encoded bytes
# params = [{'type': Types.Nat, 'value': 10}]
# data = encode(params)

# Identity and Client are dependencies of Agent
iden = Identity()  # creates a random keypair
client = Client()  # creates a client to talk to the IC
# creates an agent, combination of client and identity
agent = Agent(iden, client)


def main():
    backup(canister_id=sys.argv[1])
    schedule.every(5).seconds.do(backup, canister_id=sys.argv[1])

    while True:
        schedule.run_pending()
        time.sleep(1)


def backup(canister_id):
    print(f"disburse completed at {time.ctime()}")
    # query the NFT canister
    # doesnt change after calling `shuffleAssets`
    result = agent.update_raw(
        canister_id, "disburse", encode([]))
        
    print(result) 

get the following error after a couple of iterations:

Traceback (most recent call last):
  File "/home/moritz/.venv/bin/backup", line 8, in <module>
    sys.exit(main())
  File "/home/moritz/.venv/lib/python3.8/site-packages/state_backup/backup.py", line 25, in main
    schedule.run_pending()
  File "/home/moritz/.venv/lib/python3.8/site-packages/schedule/__init__.py", line 780, in run_pending
    default_scheduler.run_pending()
  File "/home/moritz/.venv/lib/python3.8/site-packages/schedule/__init__.py", line 100, in run_pending
    self._run_job(job)
  File "/home/moritz/.venv/lib/python3.8/site-packages/schedule/__init__.py", line 172, in _run_job
    ret = job.run()
  File "/home/moritz/.venv/lib/python3.8/site-packages/schedule/__init__.py", line 661, in run
    ret = self.job_func()
  File "/home/moritz/.venv/lib/python3.8/site-packages/state_backup/backup.py", line 33, in backup
    result = agent.update_raw(
  File "/home/moritz/.venv/lib/python3.8/site-packages/ic/agent.py", line 92, in update_raw
    return decode(result)
  File "/home/moritz/.venv/lib/python3.8/site-packages/ic/candid.py", line 1123, in decode
    b = Pipe(data)
  File "/home/moritz/.venv/lib/python3.8/site-packages/ic/candid.py", line 45, in __init__
    self._view = buffer[0:len(buffer)]
TypeError: object of type 'NoneType' has no len()