Call to undefined function arg() in cart module when user is anonomous and site caching is on

The following error is generated if the user is 'anonymous' and site caching is set to 'normal':

Fatal error: Call to undefined function arg() in /sites/all/modules/ecommerce/cart/cart.module on line 117

Although most browsers seem to get past the error and produce reasonable results, IE6 and IE7 simply truncate HTML and therefore exhibit significant rendering issues. I tested other browsers (FF2, FF3, Safari, Chrome, Konquerer) and they seem to render the page well.

There is no error after logging in.

If I set caching to 'none', there is no error, even when logged out.

It seems as if the Drupal bootstrap process skips a step for anonymous users when the site is cached and so path.inc isn't loaded when arg() is called.