无法在下拉列表中显示php

Question

我得到的错误是mysqli_fetch_assoc（）期望参数1在第18行是mysqli_result。我不太明白，因为你可以看到我的mysqli_fetch_assoc（）已经有了$ res。所以请看一下我的代码并找出问题所在。感谢

<?php
    $dbhost = 'localhost' ;
    $username = 'root' ;
    $password = '' ;
    $db = 'programmes' ;

    $link = mysqli_connect("$dbhost", "$username", "$password");

    mysqli_select_db($link, $db);

    $programme=$_GET["programme"];

if($programme!="")
{
    $res=mysqli_query($link, "select * from intakes where intake_id=$programme");
    echo "<select>";

    while($value = mysqli_fetch_assoc($res))
    {

    echo "<option>";
    echo $value["intake_list"];
    echo "</option>";
    }   
    echo "</select>";
}
    ?>