无服务器框架-尝试填充非字符串值错误

我有 serverless.yaml 对于许多SNS主题,我需要从不同帐户和地区的名称构建ARN,因此我使用了一个自定义前缀

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}

但是,当我部署它时,我不能传递错误 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.