Swagger-Codegen generates empty class instead of an instance of ByteArrayOutputStream

I'm currently trying to generate an api with swagger-codegen. The only problem i have is that swagger generates an empty class with the name "ByteArrayOutputStream" instead of an instance of ByteArrayOutputStream. The generated class also contains no attributes.

The generated swagger-yaml also defines ByteArrayOutputStream like this:

  ByteArrayOutputStream:
    type: "object"

This is the relevant part of the swagger.yaml. I've replaced some of the values with "..." and shortened the file.

---
swagger: "2.0"
info:
  version: "1.0"
  title: "..."
  contact:
    name: "..."
    url: "..."
host: "..."
basePath: "/"
schemes:
- "https"
paths:
  /rest/v1/pkw:
    post:
      summary: "..."
      description: ""
      operationId: "..."
      produces:
      - "application/json"
      parameters:
      - in: "body"
        name: "body"
        required: false
        schema:
          $ref: "#/definitions/Request"
      responses:
        200:
          description: "successful operation"
          schema:
            $ref: "#/definitions/Result"
definitions:
  ByteArrayOutputStream:
    type: "object"
  Request:
    ...
  Result:
    type: "object"
    properties:
      ...
      pdf:
        $ref: "#/definitions/ByteArrayOutputStream"