Rewrite Error when registering custom post/taxonomy?

I’ve had some code sitting inside custom_functions.php in my theme (Thesis) however when I add the code into a plugin instead it throws up a rewrite error. I’ve looked through the codex and a few posts but I can’t seem to work out what needs to change. I think it’s something to do with load order but I could be wrong?

/** Add Tutorials Post Type **/
add_action('init', 'tutorials_register');
function tutorials_register() {
    $args = array(
        'label' => __('Tutorials'),
        'singular_label' => __('Tutorial'),
        'public' => true,
        'show_ui' => true,
        'capability_type' => 'post',
        'hierarchical' => false,
        'rewrite' => true,
        'supports' => array('title', 'editor', 'author', 'thumbnail', 'excerpt', 'custom-fields', 'comments', 'revisions')
       );  

    register_post_type( 'tutorials' , $args );
}
register_taxonomy("training-course", array("tutorials"), array("hierarchical" => true, "label" => "Training Courses", "singular_label" => "Training Course", "rewrite" => true));
register_taxonomy("tutorial-category", array("tutorials"), array("hierarchical" => true, "label" => "Tutorial Categories", "singular_label" => "Tutorial Category", "rewrite" => true));
register_taxonomy("experience-level", array("tutorials"), array("hierarchical" => false, "label" => "Experience Levels", "singular_label" => "Experience Level", "rewrite" => true));
register_taxonomy("estimated-time", array("tutorials"), array("hierarchical" => false, "label" => "Estimated Completion Times", "singular_label" => "Estimated Completion Time", "rewrite" => true));

The following error message is produced when the code is activated by a plugin instead “Fatal error: Call to a member function add_rewrite_tag() on a non-object in /home/website/public_html/wp-includes/rewrite.php on line 51”

Also wondering if there is a limit on the taxonomies or if some of these would be better suited to custom fields with built in meta boxes?