Codeigniter and JQuery conflict?

[eluser]PODz_EN_Co.[/eluser]
I have a page written in php codeigniter with jquery. JSON seems to have a problem in locating the json function. Here's the scenario:

When you visit the site, you will be redirected to http://localhost/login. I have a test button in here to see if json is working well and here's the code:

JSON:

Code:

$('input#testBtn').click(function(){
    var item = $('input#txtUsername').val();
    $.post('login/testJSON', { 'item' : item },
      function(transport){ alert(transport.result); }, 'json');
});

CONTROLLER (under the login.php controller):

Code:

function testJSON(){
    $item = 'TESTJSON';
    $array = array('result' => $item);
    echo json_encode($array);
}

The above code works well if you are in the index of the login (http://localhost/login) however, if you have an invalid password you'll be redirected to http://localhost/login/authenticate which is the same page as the login index and if you try the test button for json, it will not work anymore.

Any ideas?