"channel not found" if channel is shared

[neuromusic]

We've been happily using slackin for a couple of months but recently discovered that signups are broken. A bit of trial and error indicates that the problem is that the channel we have added to "SLACK_CHANNELS" is now shared with another slack workspace.

Does slackin support signing users up for shared channels? https://api.slack.com/shared-channels#api_updates_and_additions