The provided definition does not specify a valid version field

• OS: macOS 10.12.6 (16G1510)
• Browser: chrome
• Version: 70.0.3538.110 (Official Build) (64-bit)
• Method of installation: manual
• Swagger-UI version: v3.18.3
• Swagger/OpenAPI version: 3.0

Swagger/OpenAPI definition:

openapi: "3.0.0"

servers:
  - url: 'https://domain.test/api/v1'
    description: Dev server

info:
  title: API
  description: API Documentation
  version: '1.0'

paths:
  /:
    get:
      responses:
        '200':
          description: OK

Swagger-UI configuration options:

    <script>
    window.onload = function() {
      // Begin Swagger UI call region
      const ui = SwaggerUIBundle({
        url: "{{ $ymlFileUrl }}",
        dom_id: '#swagger-ui',
        deepLinking: true,
        presets: [
          SwaggerUIBundle.presets.apis,
          SwaggerUIStandalonePreset
        ],
        plugins: [
          SwaggerUIBundle.plugins.DownloadUrl
        ],
        layout: "StandaloneLayout"
      })
      // End Swagger UI call region

      window.ui = ui
    }
  </script>

When I hit the route serving Swagger-ui I have the following message displayed:

Unable to render this definition
The provided definition does not specify a valid version field.

Please indicate a valid Swagger or OpenAPI version field. Supported version fields are swagger: "2.0" and those that match  openapi: 3.0.n (for example, openapi: 3.0.0).

When I test my definition on https://editor.swagger.io it is all good!

What am I missing?

Thank you,

J.