未捕获的PDOException：SQLSTATE [HY093]：[关闭]

错误在哪里？我看不到...

致命错误：未捕获的PDOException：SQLSTATE [HY093]：无效的参数编号：绑定变量的数量与C：\ xampp \ htdocs \ assignment7 \ login.php：15中的标记数量不匹配：堆栈跟踪：＃0 C：\ xampp \ htdocs \ assignment7 \ login.php（15）：PDOStatement-> execute（Array）＃1 C：\ xampp \ htdocs \ assignment7 \ index.php（12）：Main-> login（）＃2 {main}抛出C第15行的：\ xampp \ htdocs \ assignment7 \ login.php

function login(){
    if (isset($_POST['btn'])) {
        $user = addslashes(strip_tags($_POST['user']));
        $password = addslashes(strip_tags($_POST['password']));

        if (!empty($user) AND !empty($password)) {
            $sql = $this->_db->prepare("SELECT username, password FROM `client` WHERE username = :user AND password = :password");
            $sql->execute(array(`username` => $user, `password` => $password));

            if ($sql->rowCount()) {
                $data = $sql->fetch();
                $_SESSION['id'] = $data['id'];
                $_SESSION['id'] = true;
                header('location:client.php');
            } else {
                echo "username or password are wrong";
            }
        } else {
            echo "please enter username nad password";
        }
    }
}

my table in database