Ajax Odata 400 bad request

I get a 400 Bad Request from this code, hard to debug, what can the problem be?

        function saveSelectedAnswer(radioValue) {

            var requestUri = _spPageContextInfo.webAbsoluteUrl + "/_vti_bin/http://ift.tt/1uh5kf9";
            var listItems = {
                'Question': allQuestionsArray[0].Question,
                'Answer': radioValue,
                'QuestionID': allQuestionsArray[0].ID,
                'UserID': userGuid 
            };
            var requestBody = JSON.stringify(listItems);
            var requestHeaders = {
                "ACCEPT": "application/json;odata=verbose"
            }
            $.ajax({
                url: requestUri,
                type: "Post",
                processData: false,
                contentType: "application/json;odata=verbose",
                data: requestBody,
                headers: requestHeaders,
                success: function(data) {
                    console.log("data saved");
                },
                error: function(xhr, thrownError) {
                    alert(xhr.status + " " + thrownError);
                }
            });
        }