Issues including theme-settings.php with APC enabled

I was having trouble using Fusion (a fusion_core subtheme) with APC enabled, with an error like so:

Fatal error: require_once(). Failed opening required 'theme-settings.php'

The problem was coming from the line at the top of fusion_core's template.php:

require_once('theme-settings.php');

Changing it to the following resolved the issue.

include_once './' . drupal_get_path('theme', 'fusion_core') . '/theme-settings.php';