cache_delete with uri segments

So the manuals says to call (example);

PHP Code:

$this->db->cache_delete('blog', 'comments'); 


And it works.

However if I browse to blog/comments and do the following;

PHP Code:

$this->db->cache_delete($this->uri->segment(1), $this->uri->segment(2)); 


It returns TRUE but doesn't do anything.

(no it doesn't return anything, I changed to core a bit to return something)