CodeIgniter报错： You must use the "set" method to update an entry

I'm using codeigniter/datamapper to develop an inviocing application and I'm getting an error that i don't understand. If I do the following:

$i = new Invoice();
$i->save();

Then i get the following error: A Database Error Occurred

You must use the "set" method to update an entry.

Filename: /Users/jim/Sites/example.com/libraries/Datamapper.php

Line Number: 1635

However I can run this all day without any problem:

$i = new Invoice();
$i->notes ='x';
$i->save();

Just wondered why it behaves like this? Any help/comments much appreciated. Thanks

解答：

For the datamapper you must assign the value to the object. If the object is empty then it is not going to save. So firstly set the value for the datamapper object.

比如在用rest工具测试的时候，post请求体没有内容，或者没有设置，会导致提示这个信息