mysqli_stmt :: get_result（）不同步错误导致致命错误

Question

当我尝试这个时

    $sql = "SELECT t.fb_fullname,
        s.wins, 
        s.points FROM favorits_tbl AS f 
          INNER JOIN fb_tbl AS t ON f.favorit_id = t.user_id 
            INNER JOIN score_tbl AS s ON f.favorit_id = s.user_id 
        WHERE f.user_id = ?";

    $stmt = $this->conn->prepare($sql);
    $stmt->bind_param('s', $user_id);

    if($stmt->execute()) {
        $tasks = $stmt->get_result();
    }

我有一个

    Commands out of sync; you can't run this command now

错误。

怎么了？ 我正在使用PHP 5.6和mysqlnd驱动程序。