OpenAPI Post response 201 with content error

I want to design an API using OpenAPI v2 having returning an ID in response of a POST request. This is what I tried:

      responses:
        201:
          description: item created
          content:
            application/json:
              schema:
                type: integer
                example: 234
        400:
          description: invalid input, object invalid
        409:
          description: an existing item already exists

I am using Swagger HUB and it raises the following error for response 201:

should NOT have additional properties additionalProperty: content

>Solution :

The error occurs because the syntax you’re using (specifically, the content keyword) is OpenAPI 3.0, not 2.0.

The corresponding OpenAPI 2.0 syntax is:

    get:
      produces:
        - application/json
      ...

      responses:
        201:
          description: item created
          schema:
            type: integer
            example: 234

JSON payloads are usually sent as objects or arrays rather than primitives. I recommend that you make your response a JSON object, e.g. {"id": 234}. In this case, the response schema would look like:

      responses:
        201:
          description: item created
          schema:
            type: object
            properties:
              id:
                type: integer
                example: 234