get_terms invalid taxonomy

In my theme functions.php I have this code:

$menus = get_terms( 'nav_menu', array( 'hide_empty' => true ) );

and it works great.

I have the same code in a plugin and it says this:

object(WP_Error)#1096 (2) {
  ["errors"]=>
  array(1) {
    ["invalid_taxonomy"]=>
    array(1) {
      [0]=>
      string(16) "Invalid taxonomy"
    }
  }
  ["error_data"]=>
  array(0) {
  }
}

Why would context matter? And what can I do about it?