Warning: call_user_func(x): First argument is expected to be a valid callback in

Hello Guru’s!

I’m building an event custom post for a website using various tutorials out there. I’ve got everything I need including a metabox with a date. Although instead of a date field its giving me the following error:

Warning: call_user_func(agenda_meta_options) [function.call-user-func]: First argument is expected to be a valid callback in /home/deb45722/domains/goforthatjob.nl/public_html/wp-admin/includes/template.php on line 963

I have built an agenda custom post for another website before using the same tutorials and that worked. But it was a long time ago and I can’t remember if I had to put in a hack somewhere to show wordpress this is a valid fuction. I also can’t find a lot of information about this online using google.

Anyone know what this means?

Heres the code for my metabox:

//BEGIN METABOX
add_action('admin_init', 'register_agenda_meta');

function register_agenda_meta()
{
  add_meta_box('agenda_options', 'Item datum', 'agenda_meta_options', 'agenda', 'side', 'high');
}

function event_agenda_options()
{
  echo "<label for='start_date'>Start:</label><input type='text' name='start_date' value='' />";
}

?>