Warning: Invalid argument supplied for foreach() in 问题

菜鸟问题：
Warning: Invalid argument supplied for foreach() in F:\www\Order\index.php on line 37

Warning: Invalid argument supplied for foreach() in F:\www\Order\index.php on line 39

打开页面 提示以上问题 index.php
代码

$exts = $db->select_date('ext');
foreach($exts as $ext)
  $config['ext'][$ext['id']] = $ext;
foreach($exts as $ext)
  $config['ext'][$ext['code']] = $ext;

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$exts不是数组且不是可迭代的对象

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建议检查一下$db->select_date这个函数是否执行成功，而不是因为执行失败返回了NULL