membership code question about assigning a level

Can anyone tell me why this code would not set a user to a level in membership? The surrounding logistics work (it creates a user and everything is included in the file that needs to be), but it won't set the membership level:

$user_id = wp_insert_user($userdata);
$Membership = new M_Membership($user_id);
$level_id = 7;
$Membership->assign_level ($level_id, $fullload = true);