Fn::ImportValue does not work

[Katafalkas]

I am trying to Import bucket name:

custom:
  assets:
    auto: true # run automatically after deploy command
    targets:
      - bucket:
          Fn::ImportValue: persistent-provenance-${self:provider.stage}-MachineLearningResourcesBucketName
...

Or use variable that imports bucket name:

custom:
  mlBucketName:
    Fn::ImportValue: persistent-provenance-${self:provider.stage}-MachineLearningResourcesBucketName
  assets:
    auto: true # run automatically after deploy command
    targets:
      - bucket: ${self:custom.mlBucketName}
...

None of the options work. I get the error:

  Error: Invalid bucket name [object Object]
      at Assets.resolveBucket (/.../node_modules/serverless-s3-deploy/index.js:107:31)
      at BbPromise.map.concurrency (/.../node_modules/serverless-s3-deploy/index.js:149:23)