Fails to generate valid OpenAPI from API Blueprint

[doobnet]

I'm trying to convert an API Blueprint document to OpenAPI. No errors are generated but the resulting OpenAPI document doesn't seem to be valid.

Here's a minmal test case of an API Blueprint document that fails:

FORMAT: 1A
HOST: http://127.0.0.1/api/v1

# Group Extensions

## Extensions [/extensions]

### Get extensions [GET]

+ Response 200 (application/json)
    + Attributes (Extension)

## Data Structures

### Extension (enum)

+ (Foo type)

### Foo type (object, fixed-type)

+ id (string) - the id

The resulting OpenAPI document is:

openapi: 3.0.0
info:
  description: ''
  title: ''
  version: ''
servers:
  - url: 'http://127.0.0.1/api/v1'
paths:
  /extensions:
    get:
      responses:
        '200':
          content:
            application/json:
              examples:
                response:
                  value:
                    id: ''
              schema:
                enum:
                  - id: ''
                type: extend
          description: OK
          headers: {}
      tags:
        - Extensions
      description: ''
      summary: Get extensions
components:
  schemas:
    Extension:
      enum: []
      type: Foo type
    Extensions: {}
    Foo_type:
      properties:
        id:
          description: the id
          type: string
      type: object
tags:
  - name: Extensions

I used the following command to run api-spec-converter:

api-spec-converter -f api_blueprint -t openapi_3 -s yaml example.md > openapi.yaml

Validating the OpenAPI document using an online validating tool [1] gives the following errors:

Swagger schema validation failed.
  #/paths/~1extensions/get/responses/200/content/application~1json/schema/type must be equal to one of the allowed values
  #/paths/~1extensions/get/responses/200/content/application~1json/schema must have required property '$ref'
  #/paths/~1extensions/get/responses/200/content/application~1json/schema must match exactly one schema in oneOf
  #/paths/~1extensions/get/responses/200 must have required property '$ref'
  #/paths/~1extensions/get/responses/200 must match exactly one schema in oneOf
  #/components/schemas/Extension/enum must NOT have fewer than 1 items
  #/components/schemas/Extension/type must be equal to one of the allowed values
  #/components/schemas/Extension must have required property '$ref'
  #/components/schemas/Extension must match exactly one schema in oneOf

o@https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:95569
@https://apitools.dev/swagger-parser/online/js/bundle.min.js:29:45045

o@https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:95569
validateSchema@https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:6328
@https://apitools.dev/swagger-parser/online/js/bundle.min.js:1:3405

[1] https://apitools.dev/swagger-parser/online/