Swift POST request sends an empty body

Here's the code:

func makePOSTCall(endpoint: String, languageName: String) {
    guard let url = URL(string: endpoint) else {
        print("Could not create URL.")
        return
    }

    let requestLang: [String: Any] = ["name": languageName]
    let requestBody = try? JSONSerialization.data(withJSONObject: requestLang)

    var urlRequest = URLRequest(url: url)
    urlRequest.httpBody = requestBody
    urlRequest.httpMethod = "POST"
    let session = URLSession.shared
    let task = session.dataTask(with: urlRequest) {

        data, response, error in
        guard let data = data, error == nil else {
            print(error?.localizedDescription ?? "No data")
            return
        }

        let responseJSON = try? JSONSerialization.jsonObject(with: data, options: [])
        if let responseJSON = responseJSON as? [String: Any] {
            print(responseJSON)
        }

    }
    task.resume()
}

This sends a {"name": "Go"} JSON dictionary to Flask. Flask is supposed to append the language name to an array and return the full array in the response. Now, this works when I send the request manually, so it's not Flask's error. But when I send the above from iOS, I get request.json == None in the flask console. Clearly, I'm sending an empty body, but I shouldn't be. Any idea where I went wrong?

I call the function as

@IBAction func pressedMakePOSTCall(_ sender: UIButton) {

    makePOSTCall(endpoint: "http://127.0.0.1:5000/lang", languageName: "Go")

}

I tried adding a trailing slash, just get a 404 in the console. The only question similar to mine that I've found is this: How to make HTTP Post request with JSON body in Swift and my code is basically identical.