How send Slack message to initiator

[Nicolas Courvoisier]

Hello

I would like to verify in case of a status change to Testing, that the custom field Assigned Tester is not null. If it is null, the ticket goes back to the status Design Validation, and the user who tried to move the ticket is informed on Slack.

I tried setting the last value for "Channel or user" with the Smart Value below:

@initiator.properties.metadata.slack_id

Yes, I got the message under "Slackbot" in Slack when I tested it myself.
However, the audit log of the automated rule shows me an error "Error from Slack [404]: channel_not_found". This is annoying because it's not true (I got the message), and because this rule is red-flagged for error. 

I have to say my Webhook let me successfully send Slack messages directly to a user by using his Slack ID. 

What could be wrong here? Many thanks.