customerio-node: ERR_INVALID_ARG_TYPE: The listener argument must be of type Function on track()
This was a bug in the library itself: the track request pipeline passed a non-function where Node's EventEmitter expected a listener. The maintainer reproduced it, shipped the fix in customerio-node v3.0.3 (tested first as 3.0.3-beta.1), and the reporter confirmed the fix. Upgrade to 3.0.3 or later. Source:
Problem outcome: unknown