AppSync Arn reference error

[FAR5HID]

Trying to create an IAM role for lambda to call AppSync API from the lambda function. But having trouble with referencing the Appsync resource.

The serverless.yml (truncated):

provider:
  iam:
    role:
      statements:
        - Effect: 'Allow'
          Action:
            - 'appsync:GraphQL'
          Resource: !Sub ${appsync:arn}/types/Mutation/fields/*
resources:
  Outputs:
    apiArn:
      Value: ${appsync:arn}
appSync:
  name: AppSync-api
  ...

Used the reference variable ${appsync:arn} as mentioned in the Serverless AppSync Plugin. But running this giving me an error:

Cannot resolve serverless.yml: Variables resolution errored with:

• Cannot resolve variable at "provider.iam.role.statements.6.Resource.Fn::Sub": String value consist of variable which resolve with non-string value

If I hardcode the arn with currently deployed API arn in Resource, I get the expected arn in Outputs after successful deployment. So, how can I dynamically reference the AppSync Api Arn in IAM role statement?

[PatrykMilewski]

Duplicate off #569