这个MySQL查询和PHP foreach有什么问题

这是我的频道表：

name    url              id      key
BBC2    http://url.com/2 BBC2   2
BBC1    http://url.com/1 BBC1   1

这有效：

$getchans = "SELECT * FROM channels";  
$channels = mysqli_query($db,$getchans);    
foreach ($channels as $channel)
{
    // do stuff 
}

这不起作用：

$getchans = "SELECT * FROM channels ORDER BY key ASC";  
$channels = mysqli_query($db,$getchans);    
foreach ($channels as $channel)
{
    // do stuff 
}
并给出错误 Warning: Invalid argument supplied for foreach()

谁能告诉我为什么订购MySQL查询/结果后它不起作用？

谢谢。