medoo error return null data

Question

I am using medoo to handle table operations. And now I cannot debug via error() function of medoo at http://medoo.in/api/error.

Here is my test table structure:

CREATE TABLE [user] (
[id] INTEGER  PRIMARY KEY AUTOINCREMENT NOT NULL,
[username] NVARCHAR(50)  UNIQUE NOT NULL,
[email] NVARCHAR(50)  UNIQUE NOT NULL,
[password] VARCHAR(255)  NULL,
[displayname] NVARCHAR(255)  NULL
)

And the test code snippet:

$db = new medoo([
        'database_type' => 'sqlite',
        'database_file' => 'db/test.s3db',
    ]);
    $r = $db->insert('user', ['username' => '<existing data in table>', 'email' => '<existing data in table>']);
    var_dump($r);
    var_dump($db->error());

And I always get the array via $db->error():

[0]=> string(5) "00000" [1]=> NULL [2]=> NULL

However, as the columns are set as unique, so it should return error messages. How to fix it? Any hints are appreciated.