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我有要部署到CloudFormation的S3存储桶和策略。

Resources:
  ReportsBucket:
    Type: AWS::S3::Bucket

  BucketPolicy:
    Type: AWS::S3::BucketPolicy
    Properties:
      Bucket: !Ref ReportsBucket
      PolicyDocument:
        Id: ReportPolicy
        Version: "2012-10-17"
        Statement:
          - Sid: ReportBucketPolicyDoc
            Effect: Allow
            Action: "s3:*"
            Principal:
              AWS: !Join ['', ["arn:aws:iam::", !Ref "AWS::AccountId", ":root"]]
            Resource: !Join ['', ['arn:aws:s3:::', !Ref S3Bucket, '/*']]

它失败了，

UPDATE_ROLLBACK_IN_PROGRESS AWS::CloudFormation::Stack  {my stack name} The following resource(s) failed to create: [BucketPolicy].
CREATE_FAILED   AWS::S3::BucketPolicy   BucketPolicy    Statement is missing required element

我的政策有什么问题？