drupal_render generates php warning

I am getting this warning: warning: implode() [function.implode]: Bad arguments. in /var/www/tobaccodev/htdocs/partners/includes/form.inc on line 622.

when I use drupal_render with

$form['event_gmt'] = array (
  '#prefix' => '<div id="event_gmt"><p>*The Start and End times below are based on the time settings in your account. To see the Start and End times for another location, select the location from the list and the times will change automatically</p>',
  '#type' => 'select',
  '#options' =>  event_zonelist(),
  '#suffix' => '</div>',
);
$node->content['event_gmt'] = drupal_render($form['event_gmt']);

in my node_api hook.

any ideas why?

p.s. event_zonelist() is the function that comes with events module, and it returns an array of time zones.