cdk.FnJoin synth bug

I’m introducing an output for CloudFormation stack as follow:

new cdk.Output(this, "ApiEndpoint", {
                value: new cdk.FnJoin("", ["https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name"]),
                description: "Endpoint for this stage of the api"
            })

What I expect from the command

cdk synth

is that a code like the following is created inside the CloudFormation template:

Outputs:
    ApiEndpoint:
        Description: 'Endpoint for this stage of the api'
        Value:
            'Fn::Join':
                - ""
                -
                    - 'https://'
                    - dummy.ref
                    - .execute-api.
                    - eu-central-1
                    - .amazonaws.com/
                    - stage_name
        Export:
            Name: 'PvideoSmartCF:ApiEndpoint'

Instead the generated code is like:

Outputs:
    ApiEndpoint:
        Description: 'Endpoint for this stage of the api'
        Value:
            'Fn::Join':
                - ""
                -
                    -
                        - 'https://'
                        - dummy.ref
                        - .execute-api.
                        - eu-central-1
                        - .amazonaws.com/
                        - stage_name
        Export:
            Name: 'PvideoSmartCF:ApiEndpoint'

Which result in an error during the deployment:

failed: ValidationError: Template error: every Fn::Join object requires two parameters, (1) a string delimiter and (2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined. Template error: every Fn::Join object requires two parameters, (1) a string delimiter and (2) a list of strings to be joined or a function that returns a list of strings (such as Fn::GetAZs) to be joined.

Do anyone know how to solve this problem, or if it is a a bug or I am doing something wrong?

Thank you fro the support.

[xdodocodex]

Thank you this solved the problem

[eladb]

FnJoin accepts a variadic argument, so you would want to use it like so:

new cdk.FnJoin("", "https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name")

Alternatively, you can use the sugar FnConcat:

new cdk.FnConcat("https://","dummy.ref",".execute-api.",this.env.region,".amazonaws.com/","stage_name")