AWS SAM en Open API

Question

I would like to build an AWS API, using AWS CDK, AWS SAM en OpenAPI. As far I know, you can use new sam.CfnApi to link an OpenAPI spec to AWS SAM. But unfortunately, I got stuck in doing this.

I build my project with cdk synth, sam build and sam deploy. When I do sam deploy, I got a CREATE_FAILED with the reason:

Resource handler returned message: "Invalid OpenAPI input. (Service: ApiGateway, Status Code: 400, Request ID: 295a6724-4f91-43c5-b466-18260a1e0fab)" (RequestToken: 92104c4c-8ef5-7118-8abb-2cbdb51f5893, HandlerErrorCode: InvalidRequest)   

I use the following code to build an API..

The code I have, is this:

export class PortalXStack extends Stack {

    constructor(scope: Construct, id: string, props: PortalXStackProps) {
        super(scope, id, props);


        const data = fs.readFileSync('openapi-spec.yml');

        const restApi = new sam.CfnApi(this, 'ApiGateway2', {
            stageName: props.stageName,
            definitionBody: data
        });
    }
}

The openapi-spec.yml I have:

openapi: 3.0.3
info:
  title: Portal X
  description: Portal X
  version: 0.0.1

servers:
  - url: https://portal-x/v1

paths:
  /:
    get:
      summary: Get something
      responses:
        200:
          description: Something was ok