Sorry but where is my mistake here:
```php

	/**
	* Database object
	* @var \phpbb\db\driver\driver_interface
	*/
	protected $db;

	/**
	* Construct
	*
	* @param	\phpbb\request\request		$request	Request object
	* @param	\phpbb\template\template	$template	Template object
	* @param	\phpbb\user					$user		User object
	* @param	\phpbb\db\driver\driver_interface	$db			Database object
	* @param	string		$language_table		Table where the language strings are stored
	*/
	public function __construct(\phpbb\request\request $request, \phpbb\template\template $template, \phpbb\user $user, \phpbb\db\driver\driver_interface $db)
	{
		$this->request = $request;
		$this->template = $template;
		$this->user = $user;
		$this->db = $db;
	}

```

And this is in profilefields.yml:
```
    profilefields.type.string:
        class: phpbb\profilefields\type\type_string
        arguments:
            - @request
            - @template
            - @user
            - @dbal.conn
        tags:
            - { name: profilefield.type }
```