I am writing a program that needs to upload a file to a GitHub repo making use of the API. Using this as a reference. It looks like I have done everything correctly, but I keep getting a 400 error Problems parsing JSON upon sending a request.

I have double-checked the repo and file input,
I made sure the content is being encoded to Base64 correctly,
and the token is working on other requests so that is correct

The sha is gotten from sending a get request to the same URL and then taking the sha value of the returned json (get repository content)

function uploadFile(repo, file, contents, sha, token) {
    var body = { message: "Update", contents: btoa(contents), sha }
return fetch("https://api.github.com/repos/" + repo + "/contents" + file, { "method": "PUT", body, "headers": { "Content-Type": "application/json", "Authorization": "token " + token } })
    .then(res => res.json())
    .then(data => { return data })

}
uploadFile("My/Repo", "A/File.txt", "Test Contents", "sha", "ghp_..") //https://api.github.com/repos/My/Repo/contents/A/File.txt 400 {message: "Problems parsing JSON", documentation_url: "https://docs.github.com/rest/reference/repos#create-or-update-file-contents"}

I have tried just sending the raw string.
I also tried turning the json into a string and sending that but that returns {message: "Invalid request.↵↵"content" wasn't supplied.", documentation_url: "https://docs.github.com/rest/reference/repos#create-or-update-file-contents"}

Does anyone know what is wrong?