JSON POST headers

In a fetch for JSON POST adding these two headers:

let response = await fetch(url, {
    method: 'POST',
    body: JSON.stringify(data),
    headers:
    {
        'Content-Type': 'application/json',
        'Accept': 'text/plain', 
    }
});

returns a TypeError: Load failed message. But the target API needs both headers specified. Each header individually does not error, but the API doesn’t respond.

Any help appreciated.