Plan fails to output vaariables

[jcornadoemma]

Executing terraform plan fails with following error:

Error: Error running plan: 2 error(s) occurred:

* output.swarm_worker_token: Resource 'data.external.swarm_tokens' does not have attribute 'result.worker' for variable 'data.external.swarm_tokens.result.worker'
* output.swarm_manager_token: Resource 'data.external.swarm_tokens' does not have attribute 'result.manager' for variable 'data.external.swarm_tokens.result.manager'

Commenting following in outputs.tf works

output "swarm_manager_token" {
  value = "${data.external.swarm_tokens.result.manager}"
}

output "swarm_worker_token" {
  value = "${data.external.swarm_tokens.result.worker}"
}

Environment:
OSX High Sierra

terraform -v
Terraform v0.11.3
+ provider.external v1.0.0
+ provider.scaleway v1.0.1
+ provider.template v1.0.0