error causing php to be killed for anonymous users

If an anonymous user (uid=0) tries to access a page with any organic group content php gets killed and the following message is in the apache log:

PHP Fatal error: Call to undefined function: arg() in /home/e-smith/files/ibays/Primary/html/modules/og/og.module on line 213

No page is displayed to the anonymous user. If the user then logs in page displays fine.

If I disable the content type administration by OG module the problem resolves, so problem doesn't seem to be the OG module.