Invalid Server URL error

I don't want to put a specific url of server in OpenAPI file, as it can be used in different environments, so I use the following:
```yml
openapi: '3.0.2'
servers:
  - url: /api
```

On ```init``` method I get the error:
```
TypeError [ERR_INVALID_URL]: Invalid URL: /api
    at onParseError (internal/url.js:257:9)
    at new URL (internal/url.js:333:5)
```