$lang['is_unique'] causes "Unable to access an error message corresponding to your field name."

[eluser]Unknown[/eluser]
I'm using the next line in my validation_lang.php file:

Code:

$lang['is_unique'] = 'Uw %s is al geregisteerd.';

But when validating I get the error:
"Unable to access an error message corresponding to your field name."

All other Dutch validation messages are correctly working from the same validation_lang.php file.

The following solution does work:

Code:

$this->form_validation->set_message('is_unique', 'Uw %s is al geregisteerd.');

Can anyone explain why the first solution doesn't work?