req.body for ID is always undefined but my client side is getting the right value by jQuery .click() call

I swear it was working few days ago, so this behavior is really strange for me. Let me show you my code on client side

$(document).ready(function() {
    $("button").click(function() {
       alert(event.currentTarget.id+"------"+event.currentTarget.value);
       $.post( "/vote", {id:event.currentTarget.id,count:event.currentTarget.value}, $(this).serialize(),
           function(res) {
           }
       );
    })                  
})                      

As you can see, I tried to passed both 'id' and 'value' back to server once the button clicked. So far everything here is good because I can see the alert pop-up on screen with the correct id number and value (Output example -> aaa------23). Now take a look on my server side code.

app.post('/vote', function(req, res) {
    var id = req.body.id;
    var count = req.body.count;

    console.log("id: " + id);
    console.log("count: " + count);

    res.sendStatus(200);
    res.end();
});

When execute to the line 'var id = req.body.id', the system returns 'TypeError: Cannot read property 'id' of undefined'. 'count' will have the same result. Is there anything obvious I am missing here? Because I swear it was working before and I haven't touch this part of the code since then. Thanks for the help.

via RogerMW