S3 bucket created in a different stack is ignored

[Laurian]

I have an S3 bucket created in a different stack, and imported this way:

s3bucket:
  Fn::Join:
    - ""
    - - Fn::ImportValue: ${self:service}-storage-${self:custom.stage}-AttachmentsBucket

It does not get processed into the output file.

I guess this is similar with #52