awslabs/aws-cdk

Hi, I'm following examples of the cdk workshop here: https://github.com/aws-samples/aws-cdk-intro-workshop/blob/67c541fb1314c057e90d41804e8924845d080403/code/python/pipelines-workshop/cdk-workshop/pipeline_stack.py
I see they make the value a CfnOutput, however when I pass values from one stack to another in a pipeline let's say eg.

class CmkStack(cdk.Stack):
    def __init__(self, scope: cdk.Construct, construct_id: str, **kwargs):
        super().__init__(scope, construct_id, **kwargs)

        k = aws_kms.Key(
                self,
                'MyCMK',
                alias='my-cmk-alias',
                description='Test CMK',
                policy=policy,
                trust_account_identities=True
            )

        self._key_arn = cdk.CfnOutput(
            'KmsKey',
            description='Test CMK Output'
            value=k.key_arn
        ) 

    @property
    def key_arn(self) -> str:
        return self._key_arn


class CmkStage(cdk.Stage):
    def __init__(self, scope: cdk.Construct, construct_id: str, **kwargs):
        super().__init__(scope, construct_id, **kwargs)

        cmk = CmkStack(self, 'CMK')
        self._key_arn = cmk.key_arn

    @property
    def key_arn(self) -> str:
        return self._key_arn

and in somewhere I have a pipeline.py that says...

cmk = CMKStage(self, 'MyCMKStage')
cmk_stage = pipeline.add_application_stage(cmk)

bucket = BucketStage(self, 'BuckeStage', cmk=pipeline.stack_output(cmk.key_arn))

It resulrs in

jsii.errors.JSIIError: Expected a string, got {"$jsii.byref":"@aws-cdk/pipelines.StackOutput@10027"}

I also know that I can use SSM parameters to store values between stages however the Cdk workshop examples work I assumed this would too.