receiving error: PHP Notice: Undefined index: mode in /web/ee_web/include/form-modal.php on line 51

line 51

switch($_GET["mode"]) {

block of code it is in:

switch($_GET["mode"]) {
    case "login":
    login_user();
    break;

    case "logout":
    session_destroy();
    print "<p>You are now logged out.</p>";

    print "<p><input type=\"submit\" id=\"closeButton\" name=\"closeButton\" value=\"Close\"  onclick=\"self.parent.tb_remove(); parent.location.reload(1);\" /></p>";
    break;

    default:
    login_user();
    //print "<p>How'd you end up here?</p>";
    break;
}

The URL for the page is index.php?mode=logout, that is how I am passing the value into mode.

Any way I could resolve this error message?