Confused with set_message validation function

[eluser]CARP[/eluser]
Hi guys
I've been trying to replace default error messages when validating my forms, but I cannot get it to work.
After searching in forums, I see that the set_message function is called when using callback functions (custom validation routines) but I won't create 1 custom callback routine for just displaying my custom error message

This is my code

Code:

$rules['usernamefield']     = 'trim|required';
    $rules['emailfield']         = 'trim|required';

    $this->validation->set_message($rules['usernamefield'], 'You must fill this field!');
    $this->validation->set_message('usernamefield', 'You must fill this field!');

    $this->validation->set_rules($rules);
    $this->validation->set_error_delimiters('<span class="error">', '</span><br />');

    $fields['usernamefield']    = 'Your Username';
    $fields['emailfield']    = 'Your Email address';

    $this->validation->set_fields($fields);

but this line

Code:

$this->validation->set_message($rules['usernamefield'], 'You must fill this field!');

nor this one

Code:

$this->validation->set_message('usernamefield', 'You must fill this field!');

aren't working

Should I use set_message only when using callback custom validation functions?

Thanks a lot in adv.