AWS Api Gateway VTL Merge Json Object

Question

Trying to merge the principalId from Lambda Authorizer into the payload that will be sent to Event Bridge.

So far I've gotten close by transforming to map and re-creating the Json object. The problem is that this Json object is printed out as a key:value pair with no quotes, So there's an error thrown.

According to the docs, there's non $util.toJson($map) available, this seems to be only available for AppSync.

    const eventsAPI = new RestApi(this, 'eventsAPI', apigwOps);

    const LambdaAuth0Authorizer = new LambdaAuth0Authorizer(this, 'LambdaAuth0Authorizer', {
      env: {
        auth0Audience: '',
        auth0Domain: '',
      },
    });

    const eventTypeResource = eventsAPI.root.addResource('event');
    const options: IntegrationOptions = {
      credentialsRole: apigwRole,
      passthroughBehavior: PassthroughBehavior.NEVER,
      requestParameters: {
        'integration.request.header.X-Amz-Target': "'AWSEvents.PutEvents'",
        'integration.request.header.Content-Type': "'application/x-amz-json-1.1'",
      },

      requestTemplates: {
        'application/json': `
        #set ( $map = $util.parseJson($input.body) )
        #set ( $j = {
          "eventType": "$map['eventType']",
          "action": "$map['action']",
          "subject": "$map['subject']",
          "eventTime": $map['eventTime'],
          "actor": "$context.authorizer.principalId"
         } 
        )
                    {"Entries": 
                          [
                            {
                              "Source": "com.xyz", 
                              "Detail": "$util.escapeJavascript($j)", 
                              "Resources": [], 
                              "DetailType": "event", 
                              "EventBusName": "${eventBus.eventBusName}"
                            }
                          ]
                        }
          `,
      },
      integrationResponses: [
        {
          statusCode: '200',
          responseTemplates: {
            'application/json': '',
          },
        },
      ],
    };
    eventTypeResource.addMethod('POST', new Integration({
      type: IntegrationType.AWS,
      uri: `arn:aws:apigateway:${env.region}:events:path//`,
      integrationHttpMethod: 'POST',
      options: options,
    }), {
      authorizer: LambdaAuth0Authorizer.authorizer,
      methodResponses: [{ statusCode: '200' }],
      requestModels: { 'application/json': getEventModel(this, eventsAPI) },
      requestValidator: new RequestValidator(this, 'eventValidator', {
        restApi: eventsAPI,
        validateRequestBody: true,
      }),
    });

This generates:

{"Entries": 
  [
    {
      "Source": "com.uproarapi", 
      "Detail": "{eventType=FOLLOW, action=CREATE, subject=USER_777POIUY, eventTime=51644444444, actor=}", 
      "Resources": [], 
      "DetailType": "UpRoarEvent", 
      "EventBusName": "UpRoarEventBus"
    }
  ]
}

With an error:

{"Entries":[{"ErrorCode":"MalformedDetail","ErrorMessage":"Detail is malformed."}],"FailedEntryCount":1}