cache backup adapter?

[eluser]ipsod[/eluser]
Hi.

I initiate the caching driver like this:

Code:

$this->load->driver( 'cache', array('adapter'=>'apc', 'backup'=>'file') );

but get this error:

Fatal error: Call to undefined function apc_fetch() in /home/indig/public_html/syrupx.com/system/libraries/Cache/drivers/Cache_apc.php on line 41

I thought the 'backup'=>'file' part was supposed to fallback to file if APC wasn't on the server?