Empty response gives "Cannot read property '$ref' of undefined"

[flixx]

I have this in my openapi.yaml:

delete:
  tags:
    - Service Panel
  parameters:
    - $ref: '../../../common/parameters.yaml#/service_id'
    - $ref: '../../../common/parameters.yaml#/voucher_id'
  responses:
    204:
      description: Delete voucher
      content:
        application/json: {}

This is valid openapi 3.0 according to swagger-cli and openapi-spec-validator.

When I try to run reDoc, I get:

Cannot read property '$ref' of undefined
Stack trace
TypeError: Cannot read property '$ref' of undefined
    at e (http://127.0.0.1:8080/redoc.standalone.js:68:24962)
    at Object.n.sample (http://127.0.0.1:8080/redoc.standalone.js:68:20507)
    at e.generateExample (http://127.0.0.1:8080/redoc.standalone.js:111:40203)
    at new e (http://127.0.0.1:8080/redoc.standalone.js:111:39744)
    at http://127.0.0.1:8080/redoc.standalone.js:111:40529
    at Array.map (<anonymous>)
    at new e (http://127.0.0.1:8080/redoc.standalone.js:111:40478)
    at new e (http://127.0.0.1:8080/redoc.standalone.js:111:41426)
    at http://127.0.0.1:8080/redoc.standalone.js:111:43638
    at Array.map (<anonymous>)

ReDoc Version: 2.0.0-alpha.36 
Commit: e0d82f4

When I remove the empty response, everything works fine:

delete:
  tags:
    - Service Panel
  parameters:
    - $ref: '../../../common/parameters.yaml#/service_id'
    - $ref: '../../../common/parameters.yaml#/voucher_id'
  responses:
    204:
      description: Delete voucher