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"error: Preview failed: unknown stack" - pipeline was working last week

Are you using `StackReference`?  If so, this might be related to the note in the second paragraph here: <https://pulumi-community.slack.com/archives/CB36DSVSA/p1548728369031100>

See also <https://pulumi-community.slack.com/archives/CB36DSVSA/p1548728369031100>. 