"unknown message" when generating slack-registration.yaml

[PureTryOut]

When trying to generate slack-registration.yaml, I get a "unknown message" warning, and slack-registration.yaml is not generated.

node index.js -r -u "http://localhost:8090"
Unable to load crypto module: crypto will be disabled: Error: global.Olm is not defined
Unable to load crypto module: crypto will be disabled: Error: global.Olm is not defined
This bridge performs matrix user puppeting.
This means that the bridge logs in as your user and acts on your behalf
For the rationale, see https://github.com/kfatehi/matrix-appservice-imessage/commit/8a832051f79a94d7330be9e252eea78f76d774bc
Enter your user's localpart
 <username>
Enter password for @<username>:<server>
 ****************************************************************

Unknown message

This happens both on a Debian Stretch server and an Alpine Linux docker image, both using Node 8.12.0.