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'
  })
}

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?