Serverless Framework - Trying to populate non string value error

I have serverless.yaml with many SNS topics that I need to build ARNs from names across accounts and regions, so I come with a custom prefix

custom:
  snsTopicPrefix:
    Fn::Join:
      - ":"
      - - "arn:aws:sns"
        - !Ref "AWS::Region"
        - !Ref "AWS::AccountId"

  topic01: Topic-01-test

functions:    
  from-SNS:
    handler: src/fromSNS/handler.fromSNS
    events:
      - sns:
          arn: ${self:custom.snsTopicPrefix}:${self:custom.topic01}
          topicName: ${self:custom.topic01}

however, when I deploy it I can not pass an error Trying to populate non string value into a string for variable ${self:custom.snsTopicPrefix}. Please make sure the value of the property is a string.