"preg_grep() expects parameter 2 to be array" error

i'm running Pressflow 6.17 ...

i've installed Authcache 6x10rc2. install went ok, no errors. config seems fine.

at site/page load, i see the following in my syslog:

Jun 29 19:50:53 svr07 drupal: https://bendj.loc|1277866253|php||https://bendj.loc/||0||Warning: preg_grep() expects parameter 2 to be array, null given in _authcache_get_http_header() (line 345 of /svr/www/test03/sites/all/modules/authcache/authcache.helpers.inc).

happy to provide whatever info's needed. need to know what ... no idea what this's about.