使用Codeigniter控制器的子过滤器项目

I have a CI controller that works fine, it returns some items when I go to

mysite.com/admin/items 

It works well when I use pagination too:

mysite.com/admin/items/20, mysite.com/admin/items/40 etc. 

But, I'd like to subfilter these items with something like:

mysite.com/admin/items/starred (and to use item pagination too), 

but I'm getting Server error 500.

I've tried to use $starred = null as controller method parameter, but it dosn't work. I've tried to use routes also

$route['admin/items/starred'] = "items/starred" 

but it also doesn't work.