ky: POST request doesn't have body for Tizen 2017

I have options like this one:

const kyOptions = {
    headers: {
        'Authorization': "Bearer <token>",
    },
    json: {
        content_id: "<id>"
        content_type: "<type>"
    },
    method: "post"
};

Then I call ky:

const response = ky(uri, kyOptions);

And I have error as somehow fetch was called without body. If I replace the call with fetch itself, the request will be sent correctly as expected:

const response = await fetch(uri, {
   body: JSON.stringify(kyOptions.json),
   headers: {
      'Content-Type': 'application/json',
      ...kyOptions.headers,
   },
   method: 'POST'
});

Do you know why this can happen?

I use whatwg-fetch and core-js, maybe also something is missed?