Membership Functions not available in Admin side?

I'm trying to use current_user_on_subscription() when loading the dashboard to allow access based on membership status. If that function returns false, then they will be redirected to a subscription page.

However, that function will only return a fatal "undefined function" error.

I've tried hooking it in later in the process, but even at "shutdown" time the function still has not been defined.

I also tried to circumvent the function and call direct with...

$user = wp_get_current_user();

	$member = Membership_Plugin::factory()->get_member( $user->ID );

...which results in "Class 'Membership_Plugin' not found"

Am I wrong in thinking these functions should be available on the admin side of things?

Any ideas from the developers on this one?

Here's the full function I'm trying to use, placed in the functions.php of my active theme.

add_action( 'admin_init', 'showbiz_check_subscription' );
function showbiz_check_subscription(){
    if ( is_admin() && ! ( defined( 'DOING_AJAX' ) && DOING_AJAX ) ) {

        if (!current_user_on_subscription(1)){ // not an active subscriber

            wp_redirect( home_url('/subscribe/') );