AppSync Arn reference error

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?

sid88in / serverless-appsync-plugin

AppSync Arn reference error #600