error when creating directory service

[wooyoungAhn]

Hello
I got the error when using cdk with Python.
Template format error: The Value field of every Outputs member must evaluate to a String.
CDK version : 2.99.1 (build b2a895e)
When creating a template with the code below, output is automatically generated as shown below.
Is there anyone having the same problem as me?

code

class ADStack(NestedStack):
    def __init__(self, scope: Construct, construct_id: str, vpc, prefix, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)
        self.cfn_microsoft_AD  = directoryservice.CfnMicrosoftAD(self, "MSAD",
            name=Fn.join(delimiter=".", list_of_values=[prefix.value_as_string, "com"]),
            vpc_settings=directoryservice.CfnMicrosoftAD.VpcSettingsProperty(
                subnet_ids=[
                    vpc.select_subnets(subnet_type=ec2.SubnetType.PRIVATE_WITH_EGRESS).subnet_ids[0],
                    vpc.select_subnets(subnet_type=ec2.SubnetType.PRIVATE_WITH_EGRESS).subnet_ids[1]
                ],
                vpc_id=vpc.vpc_id
            ),
            create_alias=False,
            edition ="Standard",
            enable_sso=False,
            password="mypassword",
            short_name=prefix.value_as_string
        )
 

 "Outputs": {
  "CdkStackADStackMSADCCA41265DnsIpAddresses": {
   "Value": {
    "Fn::GetAtt": [
     "MSAD",
     "DnsIpAddresses" << it is list
    ]
   }
  }
 },