fetch http request succeeds in Chrome and Firefox but fails in Safari

If we run the following code in safari it fails saying “failed to load resource: cannot parse response”

fetch('https://mycompanyname/service/contracts-api/operatingMode', {
  method: "GET",
  headers: new Headers({
    'Authorization': 'Basic VGVzdFVzZXIxOlRFU1QhdXNlcjE=',
    'X-mycompanyname-ContractsServer': 'someurl'
  })
}

safari inspect element preview

safari inspect element headers
But if we run the same code in Chrome or Firefox, it succeeds (in the same mac computer).

Is there any quirk in Safari fetch request that I am not considering?