为什么这个准备好的语句会抛出错误？

在学习了PHP和MySQL的基础知识之后,我现在正在学习如何使用预准备语句来防止SQL注入攻击.我有以下代码:

for ($i = 0; $i < $delegateno ;$i++){

        $q = "INSERT INTO delegates (delegate_id,booker_name, booker_email, booker_tel, booker_company, delegate_name, delegate_email, delegate_tel) VALUES (NULL, ?, ?, ?, ?, ?, ?, ? )";//Insert delegate information into delegate tables
        $stmt = mysqli_query($dbc, $q);
        mysqli_stmt_bind_param($stmt,'sssssss', $fullname, $email, $tel, $company,$delegatename[$i],$delegateemail[$i],$delegatetel[$i]);
        mysqli_stmt_execute($stmt);
}

但是,这会引发:

Notice: Query: INSERT INTO delegates (delegate_id,booker_name, booker_email, booker_tel, booker_company, delegate_name, delegate_email, delegate_tel) VALUES (NULL, ?, ?, ?, ?, ?, ?, ? )
MySQL Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '?, ?, ?, ?, ?, ?, ? )'

我究竟做错了什么？