从 AWS::SSM::Document 中的参数 [StringList] 检索值

Question

我使用 CloudFormation 创建了一个 AWS SSM 文档：

SSMDocument::
Type: AWS::SSM::Document
Properties:
  DocumentType: Command
  Content:
    schemaVersion: '2.2'
    description: Run a PowerShell script
    parameters:
      Node1:
        type: String
      Node2:
        type: String
      List:
        type: StringList
    mainSteps:
    - action: aws:runPowerShellScript
      name: PreDeploy
      precondition:
        StringEquals:
        - platformType
        - Windows
      inputs:
        timeoutSeconds: '3600'
        runCommand:
        - |
          # Retrieve values from parameters
          $IpNode1 = "{{Node1}}"
          $IpNode2 = "{{Node2}}"
          $List = "{{EnvNames}}"

该文档包含PowerShell在新创建的实例上执行的脚本。我正在传递参数，其中之一($List)具有 type StringList，问题是我收到错误：

Parameter "List" is "STRING_LIST" type and can't be used as a substring parameter.

我尝试过但没有运气：

$List = "{{EnvNames}}"
$List = ["{{EnvNames}}"]
$List = @("{{EnvNames}}")