View Profiles, Friends from Subsites of WP Multisite BP Installation

I'm running WordPress Multisite with a BuddyPress installation, and this gives global options under the user profile menu on the admin bar for users to edit their profile, send messages, add friends etc.
These functions all redirect to the main BuddyPress-enabled website, which I understand is default functionality. I've read that I can enable BP_ENABLE_MULTIBLOG which fixes this and makes BP functions available across all the sites, but when I do this and try viewing a subsite, the front end gives me a 500 server error. WP admin mode works fine as does the main site.
The last line of the error log shows an incomplete line at the time of the error:
PHP message: PHP Warning: Creating default object from empty val
I'm running on Centos 6.3, php-fpm with nginx, could this be related?