[Python] pySNMP unable to dispatch SNMPv3 trap: "No callback for 'None' found"

I'm trying to get a SNMPv3 trap listener to work, and have hit a wall.

The trap packet arrives, but the code errors before the callback function ever gets called. All the available discussions point out to specifying a matching EngineID, which I have — no change.

The error is

File "/usr/local/lib/python3.9/site-packages/pysnmp/carrier/base.py", line 146, in _cbFun
raise error.CarrierError(
pysnmp.carrier.error.CarrierError: No callback for "None" found - losing incoming event


My code:

from pysnmp.hlapi import *
from pysnmp.carrier.asyncio.dispatch import AsyncioDispatcher
from pysnmp.carrier.asyncio.dgram import udp
from pyasn1.codec.ber import decoder
from pysnmp.proto import api
from pysnmp.proto import rfc1902
from pysnmp.entity import config
from pysnmp.smi import builder
from pysnmp.entity.rfc3413 import ntfrcv


snmpEngine = config.SnmpEngine()

config.addV3User(
snmpEngine,
"usertest",
config.USM_AUTH_NONE,
None,
config.USM_PRIV_NONE,
None,
securityEngineId = rfc1902.OctetString(hexValue='8000000001020304'),
)

# Callback function for handling incoming SNMP traps
def cbFun(snmpEngine, stateReference, contextEngineId, contextName, varBinds, cbCtx):
print("Made it to the callback")

transportDispatcher = AsyncioDispatcher()

ntfrcv.NotificationReceiver(snmpEngine, cbFun)

transportDispatcher.registerTransport(
udp.DOMAIN_NAME, udp.UdpAsyncioTransport().openServerMode(('192.168.120.56', 162))
)

transportDispatcher.jobStarted(1)

try:
transportDispatcher.runDispatcher()

except KeyboardInterrupt:
print("Shutting down...")

finally:
transportDispatcher.closeDispatcher()


And the command to generate the trap:

sudo snmptrap -v 3 -u usertest -l noAuthNoPriv -e 0x8000000001020304 192.168.120.56 '' .1.3.6.1.4.1


Python 3.9, pySNMP 7.1.4

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