laravel ajax 500内部服务器错误

Question

将我的AJAX从GET方法更改为POST方法后， 每次运行JavaScript时，我都会收到此内部服务器错误500。 我真的找不到任何错误。

我在窗户上使用laravel 4.2。

keyup上的JavaScript函数：

function usernameCheck()
{
var input = document.getElementById("usernameInput");
var icon = document.getElementById("userIcon");
var xmlhttp,
    username = document.getElementById("usernameInput"),
    message = document.getElementById("usernameMessage");

if(username.value != "")
{
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function()
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
            message.innerText = "";
            icon.style.color = xmlhttp.responseText;
            input.style.border = xmlhttp.responseText+" solid 2px";

            if(username.value.length < 4)
            {
                message.innerText = "username is too short";
                message.style.color = "red";
            }
            else if(xmlhttp.responseText == "false")
            {
                message.innerText = "username is already in use";
                message.style.color = "red";
            }
            else if(xmlhttp.responseText == "true")
            {
                message.innerText = "username is available";
                message.style.color = "green";
            }
        }
    }
    xmlhttp.open("POST", "usernamevalidation", true);
    xmlhttp.setRequestHeader("Content-type", "application/x-www-form-    urlencoded");
    xmlhttp.send("username="+ username.value);
}
else
{
    icon.style.color = "";
    input.style.border = "";
    message.innerText = "";
}
}

Route.php：

Route::post('usernamevalidation', 'UserController@validateUsername');