Unexpected end of json input

//Get Account Information
function GetAccountInfos()
{
    fetch("account_functions.php?action=1", {
        method: 'post', 
        headers: {
            "Content-Type": "application/json"
        }
    })
        .then(response => response.json())
        .then(data => {
            if (data !== null)
            {
                console.log(data);

                document.getElementById('account_info_username').value = data["username"];
                document.getElementById('account_info_website').value = data["website"];
                document.getElementById('account_info_email').value = data["email"];
                document.getElementById('account_info_realname').value = data["realname"];
            }
        })
        .catch(error => {
            console.error(error);
        })
}