menu_get_active_trail returns page not found

I want to add a meta description to a custom module ‘mymodule’, but this doesn’t load as expected.
The problem is in the active trail, when menu_get_active_trail is called inside array 1 the router_path = node/% and the title is page not found.
How can the active trail be fixed?
This works on other sites.

function mymodule_html_head_alter(&$head_elements) {
// menu_get_object with load function at position 2 

if ($myentity = menu_get_object('myentity', 2)) {
  dpm(menu_get_active_trail());

  $head_elements['meta_description'] = array(
    '#type' => 'html_tag',
    '#tag' => 'meta',
    '#attributes' => array(
      'name' => 'description',
      'content' => $myentity->name,
      ),
    );    
  }
}